The muscular system is responsible for the movement of the human body. Attached to the bones of the skeletal system are about 700 named muscles that make up roughly half of a person’s body weight. Each of these muscles is a discrete organ constructed of skeletal muscle tissue, blood vessels, tendons, and nerves. Muscle tissue is also found inside of the heart, digestive organs, and blood vessels. In these organs, muscles serve to move substances throughout the body.

Muscular System Anatomy

Muscle Types

There are three types of muscle tissue: Visceral, cardiac, and skeletal.

Visceral Muscle

Visceral muscle is found inside of organs like the stomach, intestines, and blood vessels. The weakest of all muscle tissues, visceral muscle makes organs contract to move substances through the organ. Because visceral muscle is controlled by the unconscious part of the brain, it is known as involuntary muscle—it cannot be directly controlled by the conscious mind. The term “smooth muscle” is often used to describe visceral muscle because it has a very smooth, uniform appearance when viewed under a microscope. This smooth appearance starkly contrasts with the banded appearance of cardiac and skeletal muscles.

Cardiac Muscle

Found only in the heart, cardiac muscle is responsible for pumping blood throughout the body. Cardiac muscle tissue cannot be controlled consciously, so it is an involuntary muscle. While hormones and signals from the brain adjust the rate of contraction, cardiac muscle stimulates itself to contract. The natural pacemaker of the heart is made of cardiac muscle tissue that stimulates other cardiac muscle cells to contract. Because of its self-stimulation, cardiac muscle is considered to be autorhythmic or intrinsically controlled.

The cells of cardiac muscle tissue are striated—that is, they appear to have light and dark stripes when viewed under a light microscope. The arrangement of protein fibers inside of the cells causes these light and dark bands. Striations indicate that a muscle cell is very strong, unlike visceral muscles.

The cells of cardiac muscle are branched X or Y shaped cells tightly connected together by special junctions called intercalated disks. Intercalated disks are made up of fingerlike projections from two neighboring cells that interlock and provide a strong bond between the cells. The branched structure and intercalated disks allow the muscle cells to resist high blood pressures and the strain of pumping blood throughout a lifetime. These features also help to spread electrochemical signals quickly from cell to cell so that the heart can beat as a unit.

Skeletal Muscle

Skeletal muscle is the only voluntary muscle tissue in the human body—it is controlled consciously. Every physical action that a person consciously performs (e.g. speaking, walking, or writing) requires skeletal muscle. The function of skeletal muscle is to contract to move parts of the body closer to the bone that the muscle is attached to. Most skeletal muscles are attached to two bones across a joint, so the muscle serves to move parts of those bones closer to each other.

Skeletal muscle cells form when many smaller progenitor cells lump themselves together to form long, straight, multinucleated fibers. Striated just like cardiac muscle, these skeletal muscle fibers are very strong. Skeletal muscle derives its name from the fact that these muscles always connect to the skeleton in at least one place.

Gross Anatomy of a Skeletal Muscle

Most skeletal muscles are attached to two bones through tendons. Tendons are tough bands of dense regular connective tissue whose strong collagen fibers firmly attach muscles to bones. Tendons are under extreme stress when muscles pull on them, so they are very strong and are woven into the coverings of both muscles and bones.

Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. One of the bones is pulled towards the other bone, which remains stationary. The place on the stationary bone that is connected via tendons to the muscle is called the origin. The place on the moving bone that is connected to the muscle via tendons is called the insertion. The belly of the muscle is the fleshy part of the muscle in between the tendons that does the actual contraction.

Names of Skeletal Muscles

Skeletal muscles are named based on many different factors, including their location, origin and insertion, number of origins, shape, size, direction, and function.

• Location. Many muscles derive their names from their anatomical region. The rectus abdominis and transverse abdominis, for example, are found in the abdominal region. Some muscles, like the tibialis anterior, are named after the part of the bone (the anterior portion of the tibia) that they are attached to. Other muscles use a hybrid of these two, like the brachioradialis, which is named after a region (brachial) and a bone (radius).
• Origin and Insertion. Some muscles are named based upon their connection to a stationary bone (origin) and a moving bone (insertion). These muscles become very easy to identify once you know the names of the bones that they are attached to. Examples of this type of muscle include the sternocleidomastoid (connecting the sternum and clavicle to the mastoid process of the skull) and the occipitofrontalis (connecting the occipital bone to the frontal bone).
• Number of Origins. Some muscles connect to more than one bone or to more than one place on a bone, and therefore have more than one origin. A muscle with two origins is called a biceps. A muscle with three origins is a triceps muscle. Finally, a muscle with four origins is a quadriceps muscle.
• Shape, Size, and Direction. We also classify muscles by their shapes. For example, the deltoids have a delta or triangular shape. The serratus muscles feature a serrated or saw-like shape. The rhomboid major is a rhombus or diamond shape. The size of the muscle can be used to distinguish between two muscles found in the same region. The gluteal region contains three muscles differentiated by size—the gluteus maximus (large), gluteus medius (medium), and gluteus minimus (smallest). Finally, the direction in which the muscle fibers run can be used to identify a muscle. In the abdominal region, there are several sets of wide, flat muscles. The muscles whose fibers run straight up and down are the rectus abdominis, the ones running transversely (left to right) are the transverse abdominis, and the ones running at an angle are the obliques.
• Function. Muscles are sometimes classified by the type of function that they perform. Most of the muscles of the forearms are named based on their function because they are located in the same region and have similar shapes and sizes. For example, the flexor group of the forearm flexes the wrist and the fingers. The supinator is a muscle that supinates the wrist by rolling it over to face palm up. In the leg, there are muscles called adductors whose role is to adduct (pull together) the legs.

Groups Action in Skeletal Muscle

Skeletal muscles rarely work by themselves to achieve movements in the body. More often they work in groups to produce precise movements. The muscle that produces any particular movement of the body is known as an agonist or prime mover. The agonist always pairs with an antagonist muscle that produces the opposite effect on the same bones. For example, the biceps brachii muscle flexes the arm at the elbow. As the antagonist for this motion, the triceps brachii muscle extends the arm at the elbow. When the triceps is extending the arm, the biceps would be considered the antagonist.

In addition to the agonist/antagonist pairing, other muscles work to support the movements of the agonist. Synergists are muscles that help to stabilize a movement and reduce extraneous movements. They are usually found in regions near the agonist and often connect to the same bones. Because skeletal muscles move the insertion closer to the immobile origin, fixator muscles assist in movement by holding the origin stable. If you lift something heavy with your arms, fixators in the trunk region hold your body upright and immobile so that you maintain your balance while lifting.

Skeletal Muscle Histology

Skeletal muscle fibers differ dramatically from other tissues of the body due to their highly specialized functions. Many of the organelles that make up muscle fibers are unique to this type of cell.

The sarcolemma is the cell membrane of muscle fibers. The sarcolemma acts as a conductor for electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules (T-tubules) that help carry these electrochemical signals into the middle of the muscle fiber. The sarcoplasmic reticulum serves as a storage facility for calcium ions (Ca2+) that are vital to muscle contraction. Mitochondria, the “power houses” of the cell, are abundant in muscle cells to break down sugars and provide energy in the form of ATP to active muscles. Most of the muscle fiber’s structure is made up of myofibrils, which are the contractile structures of the cell. Myofibrils are made up of many proteins fibers arranged into repeating subunits called sarcomeres. The sarcomere is the functional unit of muscle fibers. (See Macronutrients for more information about the roles of sugars and proteins.)

Sarcomere Structure

Sarcomeres are made of two types of protein fibers: thick filaments and thin filaments.

• Thick filaments. Thick filaments are made of many bonded units of the protein myosin. Myosin is the protein that causes muscles to contract.
• Thin filaments. Thin filaments are made of three proteins:
• Actin. Actin forms a helical structure that makes up the bulk of the thin filament mass. Actin contains myosin-binding sites that allow myosin to connect to and move actin during muscle contraction.
• Tropomyosin. Tropomyosin is a long protein fiber that wraps around actin and covers the myosin binding sites on actin.
• Troponin. Bound very tightly to tropomyosin, troponin moves tropomyosin away from myosin binding sites during muscle contraction.

Muscular System Physiology

Function of Muscle Tissue

The main function of the muscular system is movement. Muscles are the only tissue in the body that has the ability to contract and therefore move the other parts of the body.

Related to the function of movement is the muscular system’s second function: the maintenance of posture and body position. Muscles often contract to hold the body still or in a particular position rather than to cause movement. The muscles responsible for the body’s posture have the greatest endurance of all muscles in the body—they hold up the body throughout the day without becoming tired.

Another function related to movement is the movement of substances inside the body. The cardiac and visceral muscles are primarily responsible for transporting substances like blood or food from one part of the body to another.

The final function of muscle tissue is the generation of body heat. As a result of the high metabolic rate of contracting muscle, our muscular system produces a great deal of waste heat. Many small muscle contractions within the body produce our natural body heat. When we exert ourselves more than normal, the extra muscle contractions lead to a rise in body temperature and eventually to sweating.

Skeletal Muscles as Levers

Skeletal muscles work together with bones and joints to form lever systems. The muscle acts as the effort force; the joint acts as the fulcrum; the bone that the muscle moves acts as the lever; and the object being moved acts as the load.

There are three classes of levers, but the vast majority of the levers in the body are third class levers. A third class lever is a system in which the fulcrum is at the end of the lever and the effort is between the fulcrum and the load at the other end of the lever. The third class levers in the body serve to increase the distance moved by the load compared to the distance that the muscle contracts.

The tradeoff for this increase in distance is that the force required to move the load must be greater than the mass of the load. For example, the biceps brachia of the arm pulls on the radius of the forearm, causing flexion at the elbow joint in a third class lever system. A very slight change in the length of the biceps causes a much larger movement of the forearm and hand, but the force applied by the biceps must be higher than the load moved by the muscle.

Motor Units

Nerve cells called motor neurons control the skeletal muscles. Each motor neuron controls several muscle cells in a group known as a motor unit. When a motor neuron receives a signal from the brain, it stimulates all of the muscles cells in its motor unit at the same time.

The size of motor units varies throughout the body, depending on the function of a muscle. Muscles that perform fine movements—like those of the eyes or fingers—have very few muscle fibers in each motor unit to improve the precision of the brain’s control over these structures. Muscles that need a lot of strength to perform their function—like leg or arm muscles—have many muscle cells in each motor unit. One of the ways that the body can control the strength of each muscle is by determining how many motor units to activate for a given function. This explains why the same muscles that are used to pick up a pencil are also used to pick up a bowling ball.

Contraction Cycle

Muscles contract when stimulated by signals from their motor neurons. Motor neurons contact muscle cells at a point called the Neuromuscular Junction (NMJ). Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma known as the motor end plate. The motor end plate contains many ion channels that open in response to neurotransmitters and allow positive ions to enter the muscle fiber. The positive ions form an electrochemical gradient to form inside of the cell, which spreads throughout the sarcolemma and the T-tubules by opening even more ion channels.

When the positive ions reach the sarcoplasmic reticulum, Ca2+ ions are released and allowed to flow into the myofibrils. Ca2+ ions bind to troponin, which causes the troponin molecule to change shape and move nearby molecules of tropomyosin. Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together.

ATP molecules power myosin proteins in the thick filaments to bend and pull on actin molecules in the thin filaments. Myosin proteins act like oars on a boat, pulling the thin filaments closer to the center of a sarcomere. As the thin filaments are pulled together, the sarcomere shortens and contracts. Myofibrils of muscle fibers are made of many sarcomeres in a row, so that when all of the sarcomeres contract, the muscle cells shortens with a great force relative to its size.

Muscles continue contraction as long as they are stimulated by a neurotransmitter. When a motor neuron stops the release of the neurotransmitter, the process of contraction reverses itself. Calcium returns to the sarcoplasmic reticulum; troponin and tropomyosin return to their resting positions; and actin and myosin are prevented from binding. Sarcomeres return to their elongated resting state once the force of myosin pulling on actin has stopped.

Certain conditions or disorders, such as myoclonus, can affect the normal contraction of muscles. You can learn about musculoskeletal health problems in our section devoted to diseases and conditions. Also, learn more about advances in DNA health testing that help us understand genetic risk of developing early-onset primary dystonia.

Types of Muscle Contraction

The strength of a muscle’s contraction can be controlled by two factors: the number of motor units involved in contraction and the amount of stimulus from the nervous system. A single nerve impulse of a motor neuron will cause a motor unit to contract briefly before relaxing. This small contraction is known as a twitch contraction. If the motor neuron provides several signals within a short period of time, the strength and duration of the muscle contraction increases. This phenomenon is known as temporal summation. If the motor neuron provides many nerve impulses in rapid succession, the muscle may enter the state of tetanus, or complete and lasting contraction. A muscle will remain in tetanus until the nerve signal rate slows or until the muscle becomes too fatigued to maintain the tetanus.

Not all muscle contractions produce movement. Isometric contractions are light contractions that increase the tension in the muscle without exerting enough force to move a body part. When people tense their bodies due to stress, they are performing an isometric contraction. Holding an object still and maintaining posture are also the result of isometric contractions. A contraction that does produce movement is an isotonic contraction. Isotonic contractions are required to develop muscle mass through weight lifting.

Muscle tone is a natural condition in which a skeletal muscle stays partially contracted at all times. Muscle tone provides a slight tension on the muscle to prevent damage to the muscle and joints from sudden movements, and also helps to maintain the body’s posture. All muscles maintain some amount of muscle tone at all times, unless the muscle has been disconnected from the central nervous system due to nerve damage.

Functional Types of Skeletal Muscle Fibers

Skeletal muscle fibers can be divided into two types based on how they produce and use energy: Type I and Type II.

1. Type I fibers are very slow and deliberate in their contractions. They are very resistant to fatigue because they use aerobic respiration to produce energy from sugar. We find Type I fibers in muscles throughout the body for stamina and posture. Near the spine and neck regions, very high concentrations of Type I fibers hold the body up throughout the day.
2. Type II fibers are broken down into two subgroups: Type II A and Type II B.

• Type II A fibers are faster and stronger than Type I fibers, but do not have as much endurance. Type II A fibers are found throughout the body, but especially in the legs where they work to support your body throughout a long day of walking and standing.
• Type II B fibers are even faster and stronger than Type II A, but have even less endurance. Type II B fibers are also much lighter in color than Type I and Type II A due to their lack of myoglobin, an oxygen-storing pigment. We find Type II B fibers throughout the body, but particularly in the upper body where they give speed and strength to the arms and chest at the expense of stamina.

Muscle Metabolism and Fatigue

Muscles get their energy from different sources depending on the situation that the muscle is working in. Muscles use aerobic respiration when we call on them to produce a low to moderate level of force. Aerobic respiration requires oxygen to produce about 36-38 ATP molecules from a molecule of glucose. Aerobic respiration is very efficient, and can continue as long as a muscle receives adequate amounts of oxygen and glucose to keep contracting. When we use muscles to produce a high level of force, they become so tightly contracted that oxygen carrying blood cannot enter the muscle. This condition causes the muscle to create energy using lactic acid fermentation, a form of anaerobic respiration. Anaerobic respiration is much less efficient than aerobic respiration—only 2 ATP are produced for each molecule of glucose. Muscles quickly tire as they burn through their energy reserves under anaerobic respiration.

To keep muscles working for a longer period of time, muscle fibers contain several important energy molecules. Myoglobin, a red pigment found in muscles, contains iron and stores oxygen in a manner similar to hemoglobin in the blood. The oxygen from myoglobin allows muscles to continue aerobic respiration in the absence of oxygen. Another chemical that helps to keep muscles working is creatine phosphate. Muscles use energy in the form of ATP, converting ATP to ADP to release its energy. Creatine phosphate donates its phosphate group to ADP to turn it back into ATP in order to provide extra energy to the muscle. Finally, muscle fibers contain energy-storing glycogen, a large macromolecule made of many linked glucoses. Active muscles break glucoses off of glycogen molecules to provide an internal fuel supply.

When muscles run out of energy during either aerobic or anaerobic respiration, the muscle quickly tires and loses its ability to contract. This condition is known as muscle fatigue. A fatigued muscle contains very little or no oxygen, glucose or ATP, but instead has many waste products from respiration, like lactic acid and ADP. The body must take in extra oxygen after exertion to replace the oxygen that was stored in myoglobin in the muscle fiber as well as to power the aerobic respiration that will rebuild the energy supplies inside of the cell. Oxygen debt (or recovery oxygen uptake) is the name for the extra oxygen that the body must take in to restore the muscle cells to their resting state. This explains why you feel out of breath for a few minutes after a strenuous activity—your body is trying to restore itself to its normal state.

The skeletal system includes all of the bones and joints in the body. Each bone is a complex living organ that is made up of many cells, protein fibers, and minerals. The skeleton acts as a scaffold by providing support and protection for the soft tissues that make up the rest of the body. The skeletal system also provides attachment points for muscles to allow movements at the joints. New blood cells are produced by the red bone marrow inside of our bones. Bones act as the body’s warehouse for calcium, iron, and energy in the form of fat. Finally, the skeleton grows throughout childhood and provides a framework for the rest of the body to grow along with it.

Skeletal System Anatomy

The skeletal system in an adult body is made up of 206 individual bones. These bones are arranged into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones in the following regions:

• Skull
• Hyoid
• Auditory ossicles
• Ribs
• Sternum
• Vertebral column

The appendicular skeleton is made up of 126 bones in the folowing regions:

• Upper limbs
• Lower limbs
• Pelvic girdle
• Pectoral (shoulder) girdle

Skull

The skull is composed of 22 bones that are fused together except for the mandible. These 21 fused bones are separate in children to allow the skull and brain to grow, but fuse to give added strength and protection as an adult. The mandible remains as a movable jaw bone and forms the only movable joint in the skull with the temporal bone.

The bones of the superior portion of the skull are known as the cranium and protect the brain from damage. The bones of the inferior and anterior portion of the skull are known as facial bones and support the eyes, nose, and mouth.

Hyoid and Auditory Ossicles

The hyoid is a small, U-shaped bone found just inferior to the mandible. The hyoid is the only bone in the body that does not form a joint with any other bone—it is a floating bone. The hyoid’s function is to help hold the trachea open and to form a bony connection for the tongue muscles.

The malleus, incus, and stapes—known collectively as the auditory ossicles—are the smallest bones in the body. Found in a small cavity inside of the temporal bone, they serve to transmit and amplify sound from the eardrum to the inner ear.

Vertebrae

Twenty-six vertebrae form the vertebral column of the human body. They are named by region:

• Cervical (neck) – 7 vertebrae
• Thoracic (chest) – 12 vertebrae
• Lumbar (lower back) – 5 vertebrae
• Sacrum – 1 vertebra
• Coccyx (tailbone) – 1 vertebra

With the exception of the singular sacrum and coccyx, each vertebra is named for the first letter of its region and its position along the superior-inferior axis. For example, the most superior thoracic vertebra is called T1 and the most inferior is called T12.

Ribs and Sternum

The sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the thoracic region of the skeleton. The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.

There are 12 pairs of ribs that together with the sternum form the ribcage of the thoracic region. The first seven ribs are known as “true ribs” because they connect the thoracic vertebrae directly to the sternum through their own band of costal cartilage. Ribs 8, 9, and 10 all connect to the sternum through cartilage that is connected to the cartilage of the seventh rib, so we consider these to be “false ribs.” Ribs 11 and 12 are also false ribs, but are also considered to be “floating ribs” because they do not have any cartilage attachment to the sternum at all.

Pectoral Girdle and Upper Limb

The pectoral girdle connects the upper limb (arm) bones to the axial skeleton and consists of the left and right clavicles and left and right scapulae.

The humerus is the bone of the upper arm. It forms the ball and socket joint of the shoulder with the scapula and forms the elbow joint with the lower arm bones. The radius and ulna are the two bones of the forearm. The ulna is on the medial side of the forearm and forms a hinge joint with the humerus at the elbow. The radius allows the forearm and hand to turn over at the wrist joint.

The lower arm bones form the wrist joint with the carpals, a group of eight small bones that give added flexibility to the wrist. The carpals are connected to the five metacarpals that form the bones of the hand and connect to each of the fingers. Each finger has three bones known as phalanges, except for the thumb, which only has two phalanges.

Pelvic Girdle and Lower Limb

Formed by the left and right hip bones, the pelvic girdle connects the lower limb (leg) bones to the axial skeleton.

The femur is the largest bone in the body and the only bone of the thigh (femoral) region. The femur forms the ball and socket hip joint with the hip bone and forms the knee joint with the tibia and patella. Commonly called the kneecap, the patella is special because it is one of the few bones that are not present at birth. The patella forms in early childhood to support the knee for walking and crawling.

The tibia and fibula are the bones of the lower leg. The tibia is much larger than the fibula and bears almost all of the body’s weight. The fibula is mainly a muscle attachment point and is used to help maintain balance. The tibia and fibula form the ankle joint with the talus, one of the seven tarsal bones in the foot.

The tarsals are a group of seven small bones that form the posterior end of the foot and heel. The tarsals form joints with the five long metatarsals of the foot. Then each of the metatarsals forms a joint with one of the set of phalanges in the toes. Each toe has three phalanges, except for the big toe, which only has two phalanges.

Microscopic Structure of Bones

The skeleton makes up about 30-40% of an adult’s body mass. The skeleton’s mass is made up of nonliving bone matrix and many tiny bone cells. Roughly half of the bone matrix’s mass is water, while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate.

Living bone cells are found on the edges of bones and in small cavities inside of the bone matrix. Although these cells make up very little of the total bone mass, they have several very important roles in the functions of the skeletal system. The bone cells allow bones to:

• Grow and develop
• Be repaired following an injury or daily wear
• Be broken down to release their stored minerals

Types of Bones

All of the bones of the body can be broken down into five types: long, short, flat, irregular, and sesamoid.

• *Long. *Long bones are longer than they are wide and are the major bones of the limbs. Long bones grow more than the other classes of bone throughout childhood and so are responsible for the bulk of our height as adults. A hollow medullary cavity is found in the center of long bones and serves as a storage area for bone marrow. Examples of long bones include the femur, tibia, fibula, metatarsals, and phalanges.
• *Short. *Short bones are about as long as they are wide and are often cubed or round in shape. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones.
• *Flat. *Flat bones vary greatly in size and shape, but have the common feature of being very thin in one direction. Because they are thin, flat bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium—along with the ribs and hip bones—are all examples of flat bones.
• Irregular. Irregular bones have a shape that does not fit the pattern of the long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all irregular bones.
• Sesamoid. The sesamoid bones are formed after birth inside of tendons that run across joints. Sesamoid bones grow to protect the tendon from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella and the pisiform bone of the carpals are the only sesamoid bones that are counted as part of the 206 bones of the body. Other sesamoid bones can form in the joints of the hands and feet, but are not present in all people.

Parts of Bones

The long bones of the body contain many distinct regions due to the way in which they develop. At birth, each long bone is made of three individual bones separated by hyaline cartilage. Each end bone is called an epiphysis (epi = on; physis = to grow) while the middle bone is called a diaphysis (dia = passing through). The epiphyses and diaphysis grow towards one another and eventually fuse into one bone. The region of growth and eventual fusion in between the epiphysis and diaphysis is called the metaphysis (meta = after). Once the long bone parts have fused together, the only hyaline cartilage left in the bone is found as articular cartilage on the ends of the bone that form joints with other bones. The articular cartilage acts as a shock absorber and gliding surface between the bones to facilitate movement at the joint.

Looking at a bone in cross section, there are several distinct layered regions that make up a bone. The outside of a bone is covered in a thin layer of dense irregular connective tissue called the periosteum. The periosteum contains many strong collagen fibers that are used to firmly anchor tendons and muscles to the bone for movement. Stem cells and osteoblast cells in the periosteum are involved in the growth and repair of the outside of the bone due to stress and injury. Blood vessels present in the periosteum provide energy to the cells on the surface of the bone and penetrate into the bone itself to nourish the cells inside of the bone. The periosteum also contains nervous tissue and many nerve endings to give bone its sensitivity to pain when injured.

Deep to the periosteum is the compact bone that makes up the hard, mineralized portion of the bone. Compact bone is made of a matrix of hard mineral salts reinforced with tough collagen fibers. Many tiny cells called osteocytes live in small spaces in the matrix and help to maintain the strength and integrity of the compact bone.

Deep to the compact bone layer is a region of spongy bone where the bone tissue grows in thin columns called trabeculae with spaces for red bone marrow in between. The trabeculae grow in a specific pattern to resist outside stresses with the least amount of mass possible, keeping bones light but strong. Long bones have a spongy bone on their ends but have a hollow medullary cavity in the middle of the diaphysis. The medullary cavity contains red bone marrow during childhood, eventually turning into yellow bone marrow after puberty.

Articulations

An articulation, or joint, is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial fluid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and offer little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of flexibility in the joint due to the gel-like consistency of cartilage.

Skeletal System Physiology

Support and Protection

The skeletal system’s primary function is to form a solid framework that supports and protects the body’s organs and anchors the skeletal muscles. The bones of the axial skeleton act as a hard shell to protect the internal organs—such as the brain and the heart—from damage caused by external forces. The bones of the appendicular skeleton provide support and flexibility at the joints and anchor the muscles that move the limbs.

Movement

The bones of the skeletal system act as attachment points for the skeletal muscles of the body. Almost every skeletal muscle works by pulling two or more bones either closer together or further apart. Joints act as pivot points for the movement of the bones. The regions of each bone where muscles attach to the bone grow larger and stronger to support the additional force of the muscle. In addition, the overall mass and thickness of a bone increase when it is under a lot of stress from lifting weights or supporting body weight.

Hematopoiesis

Red bone marrow produces red and white blood cells in a process known as hematopoiesis. Red bone marrow is found in the hollow space inside of bones known as the medullary cavity. Children tend to have more red bone marrow compared to their body size than adults do, due to their body’s constant growth and development. The amount of red bone marrow drops off at the end of puberty, replaced by yellow bone marrow.

Storage

The skeletal system stores many different types of essential substances to facilitate growth and repair of the body. The skeletal system’s cell matrix acts as our calcium bank by storing and releasing calcium ions into the blood as needed. Proper levels of calcium ions in the blood are essential to the proper function of the nervous and muscular systems. Bone cells also release osteocalcin, a hormone that helps regulate blood sugar and fat deposition. The yellow bone marrow inside of our hollow long bones is used to store energy in the form of lipids. Finally, red bone marrow stores some iron in the form of the molecule ferritin and uses this iron to form hemoglobin in red blood cells.

Growth and Development

The skeleton begins to form early in fetal development as a flexible skeleton made of hyaline cartilage and dense irregular fibrous connective tissue. These tissues act as a soft, growing framework and placeholder for the bony skeleton that will replace them. As development progresses, blood vessels begin to grow into the soft fetal skeleton, bringing stem cells and nutrients for bone growth. Osseous tissue slowly replaces the cartilage and fibrous tissue in a process called calcification. The calcified areas spread out from their blood vessels replacing the old tissues until they reach the border of another bony area. At birth, the skeleton of a newborn has more than 300 bones; as a person ages, these bones grow together and fuse into larger bones, leaving adults with only 206 bones.

Flat bones follow the process of intramembranous ossification where the young bones grow from a primary ossification center in fibrous membranes and leave a small region of fibrous tissue in between each other. In the skull these soft spots are known as fontanels, and give the skull flexibility and room for the bones to grow. Bone slowly replaces the fontanels until the individual bones of the skull fuse together to form a rigid adult skull.

Long bones follow the process of endochondral ossification where the diaphysis grows inside of cartilage from a primary ossification center until it forms most of the bone. The epiphyses then grow from secondary ossification centers on the ends of the bone. A small band of hyaline cartilage remains in between the bones as a growth plate. As we grow through childhood, the growth plates grow under the influence of growth and sex hormones, slowly separating the bones. At the same time the bones grow larger by growing back into the growth plates. This process continues until the end of puberty, when the growth plate stops growing and the bones fuse permanently into a single bone. The vast difference in height and limb length between birth and adulthood are mainly the result of endochondral ossification in the long bones.

Diseases and Conditions

A number of musculoskeletal health issues, from arthritis to cancer, can impair our mobility and lead to loss of quality of life or even death. At other times, symptoms of joint pain can lead to diagnoses of other underlying health problems. Pay attention to joint pain and any changes you perceive in your ability to move, sharing those with your healthcare provider. Also, you can learn more about DNA health tests, which can tell you if you’re at a genetically higher risk of hemochromatosis—one of the most common hereditary disorders, causing joint pain—as well as Gaucher disease. Testing can also tell you if you’re an asymptomatic carrier of the genetic variant that you could pass along to your children.

The integumentary system is an organ system consisting of the skin, hair, nails, and exocrine glands. The skin is only a few millimeters thick yet is by far the largest organ in the body. The average person’s skin weighs 10 pounds and has a surface area of almost 20 square feet. Skin forms the body’s outer covering and forms a barrier to protect the body from chemicals, disease, UV light, and physical damage. Hair and nails extend from the skin to reinforce the skin and protect it from environmental damage. The exocrine glands of the integumentary system produce sweat, oil, and wax to cool, protect, and moisturize the skin’s surface.

Anatomy of the Integumentary System

Epidermis

The epidermis is the most superficial layer of the skin that covers almost the entire body surface. The epidermis rests upon and protects the deeper and thicker dermis layer of the skin. Structurally, the epidermis is only about a tenth of a millimeter thick but is made of 40 to 50 rows of stacked squamous epithelial cells. The epidermis is an avascular region of the body, meaning that it does not contain any blood or blood vessels. The cells of the epidermis receive all of their nutrients via diffusion of fluids from the dermis.

The epidermis is made of several specialized types of cells. Almost 90% of the epidermis is made of cells known as keratinocytes. Keratinocytes develop from stem cells at the base of the epidermis and begin to produce and store the protein keratin. Keratin makes the keratinocytes very tough, scaly and water-resistant. At about 8% of epidermal cells, melanocytes form the second most numerous cell type in the epidermis. Melanocytes produce the pigment melanin to protect the skin from ultraviolet radiation and sunburn. Langerhans cells are the third most common cells in the epidermis and make up just over 1% of all epidermal cells. Langerhans cells’ role is to detect and fight pathogens that attempt to enter the body through the skin. Finally, Merkel cells make up less than 1% of all epidermal cells but have the important function of sensing touch. Merkel cells form a disk along the deepest edge of the epidermis where they connect to nerve endings in the dermis to sense light touch.

In most of the body, the epidermis is arranged into 4 distinct layers. In the palmar surface of the hands and plantar surface of the feet, the skin is thicker than in the rest of the body and there is a fifth layer of epidermis. The deepest region of the epidermis is the stratum basale, which contains the stem cells that reproduce to form all of the other cells of the epidermis. The cells of the stratum basale include cuboidal keratinocytes, melanocytes, and Merkel cells. Superficial to stratum basale is the stratum spinosum layer where Langerhans cells are found along with many rows of spiny keratinocytes. The spines found here are cellular projections called desmosomes that form between keratinocytes to hold them together and resist friction. Just superficial to the stratum spinosum is the stratum granulosum, where keratinocytes begin to produce waxy lamellar granules to waterproof the skin. The keratinocytes in the stratum granulosum are so far removed from the dermis that they begin to die from lack of nutrients. In the thick skin of the hands and feet, there is a layer of skin superficial to the stratum granulosum known as the stratum lucidum. The stratum lucidum is made of several rows of clear, dead keratinocytes that protect the underlying layers. The outermost layer of skin is the stratum corneum. The stratum corneum is made of many rows of flattened, dead keratinocytes that protect the underlying layers. Dead keratinocytes are constantly being shed from the surface of the stratum corneum and being replaced by cells arriving from the deeper layers.

Dermis

The dermis is the deep layer of the skin found under the epidermis. The dermis is mostly made of dense irregular connective tissue along with nervous tissue, blood, and blood vessels. The dermis is much thicker than the epidermis and gives the skin its strength and elasticity. Within the dermis there are two distinct regions: the papillary layer and the reticular layer.

The papillary layer is the superficial layer of the dermis that borders on the epidermis. The papillary layer contains many finger-like extensions called dermal papillae that protrude superficially towards the epidermis. The dermal papillae increase the surface area of the dermis and contain many nerves and blood vessels that are projected toward the surface of the skin. Blood flowing through the dermal papillae provide nutrients and oxygen for the cells of the epidermis. The nerves of the dermal papillae are used to feel touch, pain, and temperature through the cells of the epidermis.

The deeper layer of the dermis, the reticular layer, is the thicker and tougher part of the dermis. The reticular layer is made of dense irregular connective tissue that contains many tough collagen and stretchy elastin fibers running in all directions to provide strength and elasticity to the skin. The reticular layer also contains blood vessels to support the skin cells and nerve tissue to sense pressure and pain in the skin.

Hypodermis

Deep to the dermis is a layer of loose connective tissues known as the hypodermis, subcutis, or subcutaneous tissue. The hypodermis serves as the flexible connection between the skin and the underlying muscles and bones as well as a fat storage area. Areolar connective tissue in the hypodermis contains elastin and collagen fibers loosely arranged to allow the skin to stretch and move independently of its underlying structures. Fatty adipose tissue in the hypodermis stores energy in the form of triglycerides. Adipose also helps to insulate the body by trapping body heat produced by the underlying muscles.

Hair

Hair is an accessory organ of the skin made of columns of tightly packed dead keratinocytes found in most regions of the body. The few hairless parts of the body include the palmar surface of the hands, plantar surface of the feet, lips, labia minora, and glans penis. Hair helps to protect the body from UV radiation by preventing sunlight from striking the skin. Hair also insulates the body by trapping warm air around the skin.

The structure of hair can be broken down into 3 major parts: the follicle, root, and shaft. The hair follicle is a depression of epidermal cells deep into the dermis. Stem cells in the follicle reproduce to form the keratinocytes that eventually form the hair while melanocytes produce pigment that gives the hair its color. Within the follicle is the hair root, the portion of the hair below the skin’s surface. As the follicle produces new hair, the cells in the root push up to the surface until they exit the skin. The hair shaft consists of the part of the hair that is found outside of the skin.

The hair shaft and root are made of 3 distinct layers of cells: the cuticle, cortex, and medulla. The cuticle is the outermost layer made of keratinocytes. The keratinocytes of the cuticle are stacked on top of each other like shingles so that the outer tip of each cell points away from the body. Under the cuticle are the cells of the cortex that form the majority of the hair’s width. The spindle-shaped and tightly packed cortex cells contain pigments that give the hair its color. The innermost layer of the hair, the medulla, is not present in all hairs. When present, the medulla usually contains highly pigmented cells full of keratin. When the medulla is absent, the cortex continues through the middle of the hair.

Nails

Nails are accessory organs of the skin made of sheets of hardened keratinocytes and found on the distal ends of the fingers and toes. Fingernails and toenails reinforce and protect the end of the digits and are used for scraping and manipulating small objects. There are 3 main parts of a nail: the root, body, and free edge. The nail root is the portion of the nail found under the surface of the skin. The nail body is the visible external portion of the nail. The free edge is the distal end portion of the nail that has grown beyond the end of the finger or toe.

Nails grow from a deep layer of epidermal tissue known as the nail matrix, which surrounds the nail root. The stem cells of the nail matrix reproduce to form keratinocytes, which in turn produce keratin protein and pack into tough sheets of hardened cells. The sheets of keratinocytes form the hard nail root that slowly grows out of the skin and forms the nail body as it reaches the skin’s surface. The cells of the nail root and nail body are pushed toward the distal end of the finger or toe by new cells being formed in the nail matrix. Under the nail body is a layer of epidermis and dermis known as the nail bed. The nail bed is pink in color due to the presence of capillaries that support the cells of the nail body. The proximal end of the nail near the root forms a whitish crescent shape known as the lunula where a small amount of nail matrix is visible through the nail body. Around the proximal and lateral edges of the nail is the eponychium, a layer of epithelium that overlaps and covers the edge of the nail body. The eponychium helps to seal the edges of the nail to prevent infection of the underlying tissues.

Sudoriferous Glands

Sudoriferous glands are exocrine glands found in the dermis of the skin and commonly known as sweat glands. There are 2 major types of sudoriferous glands: eccrine sweat glands and apocrine sweat glands. Eccrine sweat glands are found in almost every region of the skin and produce a secretion of water and sodium chloride. Eccrine sweat is delivered via a duct to the surface of the skin and is used to lower the body’s temperature through evaporative cooling.

Apocrine sweat glands are found in mainly in the axillary and pubic regions of the body. The ducts of apocrine sweat glands extend into the follicles of hairs so that the sweat produced by these glands exits the body along the surface of the hair shaft. Apocrine sweat glands are inactive until puberty, at which point they produce a thick, oily liquid that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria produces body odor.

Sebaceous Glands

Sebaceous glands are exocrine glands found in the dermis of the skin that produce an oily secretion known as sebum. Sebaceous glands are found in every part of the skin except for the thick skin of the palms of the hands and soles of the feet. Sebum is produced in the sebaceous glands and carried through ducts to the surface of the skin or to hair follicles. Sebum acts to waterproof and increase the elasticity of the skin. Sebum also lubricates and protects the cuticles of hairs as they pass through the follicles to the exterior of the body.

Ceruminous Glands

Ceruminous glands are special exocrine glands found only in the dermis of the ear canals. Ceruminous glands produce a waxy secretion known as cerumen to protect the ear canals and lubricate the eardrum. Cerumen protects the ears by trapping foreign material such as dust and airborne pathogens that enter the ear canal. Cerumen is made continuously and slowly pushes older cerumen outward toward the exterior of the ear canal where it falls out of the ear or is manually removed.

Physiology of the Integumentary System

Keratinization

Keratinization, also known as cornification, is the process of keratin accumulating within keratinocytes. Keratinocytes begin their life as offspring of the stem cells of the stratum basale. Young keratinocytes have a cuboidal shape and contain almost no keratin protein at all. As the stem cells multiply, they push older keratinocytes towards the surface of the skin and into the superficial layers of the epidermis. By the time keratinocytes reach the stratum spinosum, they have begun to accumulate a significant amount of keratin and have become harder, flatter, and more water resistant. As the keratinocytes reach the stratum granulosum, they have become much flatter and are almost completely filled with keratin. At this point the cells are so far removed from the nutrients that diffuse from the blood vessels in the dermis that the cells go through the process of apoptosis. Apoptosis is programmed cell death where the cell digests its own nucleus and organelles, leaving only a tough, keratin-filled shell behind. Dead keratinocytes moving into the stratum lucidum and stratum corneum are very flat, hard, and tightly packed so as to form a keratin barrier to protect the underlying tissues.

Temperature Homeostasis

Being the body’s outermost organ, the skin is able to regulate the body’s temperature by controlling how the body interacts with its environment. In the case of the body entering a state of hyperthermia, the skin is able to reduce body temperature through sweating and vasodilation. Sweat produced by sudoriferous glands delivers water to the surface of the body where it begins to evaporate. The evaporation of sweat absorbs heat and cools the body’s surface. Vasodilation is the process through which smooth muscle lining the blood vessels in the dermis relax and allow more blood to enter the skin. Blood transports heat through the body, pulling heat away from the body’s core and depositing it in the skin where it can radiate out of the body and into the external environment.

In the case of the body entering a state of hypothermia, the skin is able to raise body temperature through the contraction of arrector pili muscles and through vasoconstriction. The follicles of hairs have small bundles of smooth muscle attached to their base called arrector pili muscles. The arrector pili form goose bumps by contracting to move the hair follicle and lifting the hair shaft upright from the surface of the skin. This movement results in more air being trapped under the hairs to insulate the surface of the body. Vasoconstriction is the process of smooth muscles in the walls of blood vessels in the dermis contracting to reduce the flood of blood to the skin. Vasoconstriction permits the skin to cool while blood stays in the body’s core to maintain heat and circulation in the vital organs.

Vitamin D Synthesis

Vitamin D, an essential vitamin necessary for the absorption of calcium from food, is produced by ultraviolet (UV) light striking the skin. The stratum basale and stratum spinosum layers of the epidermis contain a sterol molecule known as 7-dehydrocholesterol. When UV light present in sunlight or tanning bed lights strikes the skin, it penetrates through the outer layers of the epidermis and strikes some of the molecules of 7-dehydrocholesterol, converting it into vitamin D3. Vitamin D3 is converted in the kidneys into calcitriol, the active form of vitamin D. When our skin is not exposed to sufficient amounts of sunlight, we can develop vitamin D deficiency, potentially leading to serious health concerns. The ability to order a vitamin D home test and check our own levels thankfully makes it simpler to identify deficiency.

Protection

The skin provides protection to its underlying tissues from pathogens, mechanical damage, and UV light. Pathogens, such as viruses and bacteria, are unable to enter the body through unbroken skin due to the outermost layers of epidermis containing an unending supply of tough, dead keratinocytes. This protection explains the necessity of cleaning and covering cuts and scrapes with bandages to prevent infection. Minor mechanical damage from rough or sharp objects is mostly absorbed by the skin before it can damage the underlying tissues. Epidermal cells reproduce constantly to quickly repair any damage to the skin. Melanocytes in the epidermis produce the pigment melanin, which absorbs UV light before it can pass through the skin. UV light can cause cells to become cancerous if not blocked from entering the body.

Skin Color

Human skin color is controlled by the interaction of 3 pigments: melanin, carotene, and hemoglobin. Melanin is a brown or black pigment produced by melanocytes to protect the skin from UV radiation. Melanin gives skin its tan or brown coloration and provides the color of brown or black hair. Melanin production increases as the skin is exposed to higher levels of UV light resulting in tanning of the skin. Carotene is another pigment present in the skin that produces a yellow or orange cast to the skin and is most noticeable in people with low levels of melanin. Hemoglobin is another pigment most noticeable in people with little melanin. Hemoglobin is the red pigment found in red blood cells, but can be seen through the layers of the skin as a light red or pink color. Hemoglobin is most noticeable in skin coloration during times of vasodilation when the capillaries of the dermis are open to carry more blood to the skin’s surface.

Cutaneous Sensation

The skin allows the body to sense its external environment by picking up signals for touch, pressure, vibration, temperature, and pain. Merkel disks in the epidermis connect to nerve cells in the dermis to detect shapes and textures of objects contacting the skin. Corpuscles of touch are structures found in the dermal papillae of the dermis that also detect touch by objects contacting the skin. Lamellar corpuscles found deep in the dermis sense pressure and vibration of the skin. Throughout the dermis there are many free nerve endings that are simply neurons with their dendrites spread throughout the dermis. Free nerve endings may be sensitive to pain, warmth, or cold. The density of these sensory receptors in the skin varies throughout the body, resulting in some regions of the body being more sensitive to touch, temperature, or pain than other regions.

Excretion

In addition to secreting sweat to cool the body, eccrine sudoriferous glands of the skin also excrete waste products out of the body. Sweat produced by eccrine sudoriferous glands normally contains mostly water with many electrolytes and a few other trace chemicals. The most common electrolytes found in sweat are sodium and chloride, but potassium, calcium, and magnesium ions may be excreted as well. When these electrolytes reach high levels in the blood, their presence in sweat also increases, helping to reduce their presence within the body. In addition to electrolytes, sweat contains and helps to excrete small amounts of metabolic waste products such as lactic acid, urea, uric acid, and ammonia. Finally, eccrine sudoriferous glands can help to excrete alcohol from the body of someone who has been drinking alcoholic beverages. Alcohol causes vasodilation in the dermis, leading to increased perspiration as more blood reaches sweat glands. The alcohol in the blood is absorbed by the cells of the sweat glands, causing it to be excreted along with the other components of sweat.

For many, college is a time of burgeoning sexual self-actualization, exploration, and experimentation. A recent report from the National College Health Association found that more than half of college students report having sex in the 12 months preceding the survey.¹ Most students are keenly aware of this going into college, which can add a lot of pressure on students regardless of their experience level. And that pressure can lead to poor judgment and a cascade of unpleasant consequences.

Knowing all you can about collegiate sexual activity, expectations, and best practices allows you to maximize your safety without dampening that pioneering spirit, ensuring that your memories are the only things from college that follow you for life.

This guide will give you useful information to keep you safe and let you have experiences that can positively shape your time in college.

Normal sexual activity among college students

Questions of normalcy abound in sexual health both on and off college campuses. But for students who may have minimal or no experience with sex, it’s hard to know how your activity compares to the average.

We want to stress that there is no “normal” when it comes to sex and that you shouldn’t compare your experiences to others in a way that diminishes either party. That said, there are statistical realities about recurrent behaviors among college students that may give you some comfort if you’re worried that you don’t measure up.

According to a CDC report from 2023, around one in three high school students will have had sex before graduation.² That means a full two-thirds of students entering college have yet to experience sexual intercourse.

Here are the most prevalent sexual behaviors among college students:³

• Solo masturbation: 88.6%
• Oral sex (received): 79.4%
• Oral sex (performed): 78.4%
• Penile-vaginal intercourse: 73.5%
• Partnered masturbation: 71.1%
• Anal intercourse (received): 16.8%
• Anal intercourse: (performed): 25.3%
• Choking (received): 43.0%
• Choking (performed): 47.3%
• Spanking (received): 59.1%
• Facial slapping (received): 12.1%

The prevalence of choking⁴ may surprise some, but it’s backed up by additional studies that come to similar conclusions.

Alcohol and sex in college

According to the NIH, at least 49% of college students drink alcohol at least once monthly, with 29% binge drinking.⁵ Both numbers are down a few points from previous years, but the real numbers are likely higher since these are self-reported survey statistics. So, there’s not much point in proclaiming the dangers of alcohol abuse, however valid those proclamations are. College students are more likely to drink than not.

We encourage you not to engage in sex when intoxicated — especially with new partners. You might neglect to use protection or effectively communicate consent. You might even miss signs of active STDs like genital warts that might send you running in the other direction if you were sober.

None of this is to shame anyone for mistakes made at college under the influence of drugs or alcohol. Frankly, shame is unhealthy and unhelpful in this context.⁶ The above only serves to illustrate undeniable risks.

But if you or a fellow student are among the 14% of college students who meet the criteria for alcohol use disorder,⁵ there are some excellent resources for you, including:

• Alcohol Rehab Guide
• Alcohol Rehab Help
• College Drinking Prevention
• Rehab Spot

STDs and STIs on college campuses

One of the biggest risks you take engaging in sex on a college campus is contracting a sexually transmitted disease. In fact, one in four college students has an active STI,⁷ meaning there’s a 25% chance that someone you sleep with in college could potentially transmit something to you. And by age 25, one in two people will have had an STI at some point.

This is despite data that illustrate a slowdown in sexual behaviors among college students — a so-called sexual recession.⁸

The most common STDs on college campuses greatly resemble the most common STDs in the general populace. The big difference is that those aged 15-24 consistently make up the majority of new cases.

Here are the most prevalent STDs on college campuses:⁹

• Herpes (HSV-I): 60%¹⁰
• EBV: 56%¹¹
• HPV: >43%¹²
• Chlamydia: 3%¹³
• Gonorrhea: 2.1%¹⁴
• Trichomoniasis: 2.3%¹⁵
• Herpes (HSV-II): 1.6%¹⁶
• HIV 0.5%:¹⁷
• Syphilis: 0.03%¹⁸

STDs vs. STIs

You’ll often hear the terms STD and STI bandied about somewhat loosely, but the difference is simple. STI stands for sexually transmitted infection, and it applies to anyone who contracts enough viral or bacterial cells from a partner to become infected. The medical community doesn’t consider an STI to be an STD until a patient presents symptoms.¹⁹

This may seem like splitting hairs, but it serves a medical purpose in maintaining clarity in a diagnosis. For a college student, either term is acceptable, and you want to avoid encounters with both.

Protecting yourself from STDs

There are several ways you can protect yourself from STDs. Each has its own success rate, and it’s ultimately up to you how much of a risk you’re willing to take with your health. But from physical discomfort to mortal peril, there is a wide range of reasons to avoid STIs.

Here are the best ways to protect yourself from infection:

Condoms

We often think of condoms as a reliable protector against pregnancy and STIs. They are highly effective (up to 98%) against pregnancy and STIs like chlamydia, gonorrhea, trichomoniasis, and HIV.²⁰ However, condoms don’t cover the entire genital area, so they can fail to stop the skin-to-skin spread of diseases like syphilis, genital warts, and herpes.

Dental dams

Dental dams are probably among the least popular safe sex tools in existence. Unlike condoms, which have been around since the 1600s, dental dams have only been part of the STD prevention landscape since the late 1980s. And one study out of Australia indicates that less than 10% of women ever use them.²¹ They are likely more effective against transmission in cases of fellatio than cunnilingus. The one upside is that you can DIY some for cheap that are just as effective: just cut a piece of plastic wrap to an appropriate size.

Latex Underwear

While latex underwear has been around in the BDSM world and elsewhere for some time, the FDA recently approved two styles of latex panties for STI prevention during cunnilingus and analingus. The panties come from a company called Lorals, and they’re available in bikini and shortie styles.

Mutual monogamy

Mutual monogamy is all about trust. If you trust your partner is faithful to you, you have less to worry about in the STD department. That said, you should always get tested at the outset of a new relationship and insist any new partners do, as well. That ensures that you aren’t carrying anything asymptomatically.

Vaccination

There aren’t many STDs against which you can be vaccinated, but HPV is one of them. The HPV vaccine has evolved to include additional dangerous variants in the past few years. So even if you got the vaccine in your youth, you may be eligible for the updated vaccine, as there is no conflict between the two.

Abstinence

This is one of the less realistic strategies for college students, but it’s the most reliable. Still, it isn’t perfect. Even abstinent individuals can contract STIs in some unexpected ways. The Epstein-Barr virus, which often leads to mononucleosis,²² spreads via saliva. Deep kissing is often a major cause of its transmission.

Testing

Regular testing can help you identify an STD before you spread it to anyone else. This is a bit less about protecting yourself and more about creating a community where we can all minimize the spread of STDs. If you’re worried about being seen at a campus clinic, you should investigate many of the available at-home STD tests on the market.

Suppose you need a little more inspiration to take active steps to protect yourself from STDs. In that case, the CDC maintains a set of relatively gruesome picture cards showing various unpleasant physical symptoms of various STDS. Feel free to look at them if you ever need a boost.

According to a study published in the American Journal of College Health, college students were less likely than non-students to lose their virginity at a younger age, have multiple sexual partners, or have had sex without using a condom.²⁷ Given these stats, the likelihood of a collegiate hookup resulting in the transmission of an STI might be lower than it is for the rest of the populace, but that’s no reason to ignore risks or skimp on testing.

How to know if you have an STI

There are various signs that you may have been infected with an STI. What’s more worrisome is that just about every STD can be asymptomatic for its lifespan. That means you could unknowingly spread a potentially deadly disease or experience late-term symptoms years down the line.

Here’s a look at some common symptoms to watch out for, as well as whether a given infection may be asymptomatic.

	Common symptoms	May be asymptomatic
Chlamydia	Painful urination, genital discharge
Gonorrhea	Genital discharge, persistent sore throat
Trich	Genital itching, burning, or redness
HPV	Genital warts, cervical lesions
Herpes	Genital, oral, or anal sores
Syphilis	Rash, hair loss, muscle aches
HIV	Fever, swollen lymph nodes

When to get tested for an STI

Testing is one of the essential tools for identifying STIs and limiting their spread. It might seem scary, but the methods used today are a lot less invasive than those used in the past, so there’s little to worry about in terms of the testing experience.

Knowing when to get tested can be confusing, especially if you’re not that active and don’t show any STI symptoms.

• Get tested immediately if you show signs or experience symptoms of an STI.
• Get tested at least every 3-6 months if you’re sexually active but not in a monogamous relationship.
• Get tested at least once per year if you’re sexually active and in a monogamous relationship.
• Get tested within an appropriate window period if you know you’ve been exposed to a particular STD. Refrain from intercourse until your results come in.
• Get tested quickly after having unprotected sex. You may want to take more than one test to cover various window periods. Refrain from intercourse until you get your results.

What are window periods?

Not all STIs are detectable within the same amount of time from exposure. A window period is the amount of time between exposure and detectable infection. For example, you could have unprotected sex with someone infected with gonorrhea and want to take a test immediately after finding out they’re infected.

It could take between five days and two weeks for a test to be positive, even if you are infected. That means you should wait two weeks from the exposure to take a test. It also means you’ll have to refrain from sex until those results come back, including sex with the infected partner. You may not have contracted it the first time, and there’s no reason to increase the odds that you will.

Here are the different window periods for common STIs:

• HPV: 2-3 months
• Chlamydia: Five days to two weeks
• Gonorrhea: Five days to two weeks
• Syphilis: 1-3 months
• Herpes (HSV-I): 4-6 weeks
• Herpes(HSV-II): 4-6 weeks
• Trichomoniasis: 3-7 days
• HIV: 23-90 weeks

STD treatment resources

College campuses generally take their STD rates seriously and invest heavily in their on-campus health clinics. You can get tested for most STIs and receive treatment through on-campus clinics at most colleges.

Some non-secular colleges and universities may have restrictions regarding sexual health and safety,²³ preferring instead to push abstinence. If you attend such a school and still need help, there are likely local resources that can help. Tools like the National Association of Free and Charitable Clinics’ clinic finder can help you find resources within a reasonable distance.

You should take any potential exposure seriously, even if the infection is bacterial and especially if you’re asymptomatic after exposure. Left untreated, many STDs can result in infertility or worse. Here’s a look at some potential long-term side effects of untreated STDs:

	Curable?	Treatment	Risk if left untreated
Chlamydia		Antibiotics	Pelvic inflammatory disease, infertility
Gonorrhea		Antibiotics	Infertility
Trich		Antibiotics	Infertility
HPV		Vaccination, wart creams	Cervical, genital, or oral cancer
Herpes		Prescriptions to limit outbreaks	Increased risk of other STDs
Syphilis		Penicillin and antibiotics	Dementia, internal bleeding, death
HIV		Anti-virals	AIDS, death

Accessing birth control

Access to birth control is in constant flux, depending largely on the state in which you live. Federal regulations might further complicate the picture moving forward, but for now, access to birth control for some students may be as simple as walking over to your campus clinic or so complicated that you’re tempted to give up the search.

As with STD resources, the best place to start is at your campus clinic. If that fails, Planned Parenthood may have local clinics and resources to help you access birth control. Planned Parenthood also offers advice and resources for obtaining emergency contraception after unprotected sex.

Telemedicine has made it exceedingly simple for some people to acquire a prescription for and free delivery of birth control pills, so long as current regulations stand. You can also use a helpful clinic finder and appointment tool from Options for Sexual Health.

Some online services will ship birth control to you in discreet packaging, which is especially useful if you live in a contraceptive desert. These companies include Favor (previously Pill Club), Simple Health, and Nurx. Different organizations may also carry other sexual health goods that are hard to come by, like emergency contraception (Plan B or Ella), internal condoms, and herpes medication.

Importance of consent

There may be nothing more important in a sexual relationship than consent. This is as true off-campus as it is on-campus. There shouldn’t be any gray area here, either. That doesn’t mean that you need to get written permission from each other to have sex, but it does mean that verbal consent is the gold standard.

If you have any reason to suspect your partner isn’t enthusiastically on board with the direction your interaction is taking, you should simply ask. If you’re worried it will ruin the mood, it won’t. If it does, then that’s not someone with whom you want to have a sexual relationship.

Of course, explicit consent becomes even more critical when you consider the rates of drug and alcohol use among college students. According to a 2023 report from the Substance Abuse and Mental Health Services Administration (SAMHSA), nearly 49.6% of individuals aged 18-25 consumed alcohol within the past month.²⁴ These are based on self-reported statistics; the real percentage may be higher.

Whether cold sober, slightly buzzed, or blackout drunk, sexual consent is an absolute must. The problem here is that alcohol commonly reduces inhibitions and impairs judgment.

Here are some signs²⁵ to look for that might indicate your potential partner is not in a sober enough position to give consent:

• Slurred speech
• Trouble walking
• Poor coordination
• Heavy eyes
• Confusion
• Sweating
• Vomiting
• Slow reaction time

There are many more indications that someone may be too intoxicated for consent. There are also instances in which someone is too intoxicated for consent but otherwise seems normal. This is why clear consent is so important. Never assume that your partner is equally enthusiastic, especially when drugs or alcohol are present.

Sexual assault on college campuses

Unfortunately, consent is not something everyone holds dear. There are far more sexual acts performed without consent — or with a crystal clear refusal of consent — than most people would assume. Women on college campuses have three times the risk of sexual assault as women in general. And a shocking 13% of college students experience rape or sexual assault (including assault during periods of incapacitation).²⁶

It’s not just women who face the threat of sexual violence, either. Male-identifying college students are 78% more likely to endure rape or sexual assault than their peers outside of college.

Here are some other important RAINN (Rape, Abuse, and Incest National Network) statistics to consider:

• Nearly a quarter of trans, genderqueer, and nonconforming students have been sexually assaulted.
• Nearly 6% of students experience stalking during their time at college.
• Only one out of five female survivors have had assistance through a victims’ services agency.
• Only about 20% of female survivors file a report of sexual assault with law enforcement.

Assistance and reporting resources

Part of getting survivors the assistance they need — including access to justice — is expanding the knowledge of available resources. If you’ve endured any form of sexual violence, there are various resources at your disposal. Some provide physical and psychological support, while others can help you file charges against assailants within criminal and collegiate justice systems.

RAINN

RAINN’s National Sexual Assault Hotline offers support for survivors of sexual assault anywhere in the nation — on college campuses and beyond.

RAINN’s contact resources include:

• Phone support: 800-656-HOPE (4673)
• Online chat (English)
• Online chat (Español)

RAINN also has a local service provider tool to help you find help centers in your immediate vicinity.

1in6

The name 1in6 is a direct reference to the portion of men who experience sexual assault. The organization offers various resources for men, not least of which involves busting myths and resolving the stigma around male victimhood.

1in6’s resources include:

• Anonymous, counselor-facilitated group sessions
• One-on-one chat through Survivor Space
• Myths and facts about male abuse and assault
• Book and film recommendations

National Sexual Violence Resource Center

This invaluable resource provides a wide range of information on sexual assault for victims, family members, and more. They also host advocacy and education events around the country.

Some of their most important resources include:

• Directory of organizations
• Mental health resources for black, indigenous, and people of color (BIPOC) survivors
• Trauma exploration blog
• Guide for family and friends of survivors

Best practices for safe sex in college

There are many new things to learn when you get to college, not least of all what your classes have to teach you: a new landscape, new friends and neighbors, and new responsibilities for starters. We’ve put together this set of best practices you can follow to make the safe sex side of the equation a bit easier to take in. The list is by no means exhaustive, but it’s a great place to start. Hopefully, it contains some valuable information you didn’t have before.

Learn your limits

You’ll try a lot of new things when you get to college, and learning where your personal limits lie is critical. What might you be willing to do drunk that you wouldn’t do sober? If you consume alcohol, how quickly do you become drunk? If you do drugs, how do they affect you? You can put all of this information together to help you realize when you’re about to make a decision that’s not in line with your sober thinking. It can also keep you from overindulging in any substance that could impair your judgment beyond safety.

If you’re having a hard time keeping within your limits, you may want to speak to someone about your alcohol and drug use. The College Alcohol Intervention Matrix has a handful of excellent resources that can help you identify issues and provide you with treatment.

Get tested often

Testing is the best way to ensure that you and your fellow campus-mates are safe. Knowing you have a clean bill of health will also keep you from getting to the point where you’re ready to enjoy the company of a classmate, only for them to ask you about your sexual health status and derail the whole relationship. And since so many potentially deadly STDs can exist inside you asymptomatically, knowing you have an infection could save your life.

Everyone wins when you test regularly.

Talk to your partners

It’s not easy to stop in the middle of foreplay and ask a potential partner if they’ve been tested lately, but that’s no reason not to do it. Talking to your partners before and after sex ensures that you’re always on the same page about sexual health and consent.

Planned Parenthood offers some great advice on talking to your partner about sexual health.

Investigate your campus’s resources

Most college campuses have a litany of resources dedicated to safe sex and sexual security. You may learn about these during your orientation, and your college’s website likely has a section devoted to them. If you’re sexually active on your campus or thinking of increasing your sexual activity, you should familiarize yourself with these resources as much as possible.

Use the right condoms, and use them correctly

The condom industry largely takes a one-size-fits-all approach to its products, resulting in unintentional misuse and even embarrassment. Some companies offer custom-sized condoms through online interfaces, which can help if you land in a size between specialty small, standard, and magnum sizes.

And please use your condoms correctly. That doesn’t just mean putting them on correctly, either. It means storing them correctly.

Where not to store your condoms:

• Back pockets
• Wallet
• Car (glovebox, center console, etc.)
• Fridge or freezer
• Outdoors

Where to safely store your condoms:

• Uncluttered drawers (ideally bedside)
• Empty tin
• Toiletry or cosmetic bag (without sharp objects in it)
• Front pocket for no more than a few hours

Use the right lube for the situation at hand

Lubrication can make an incredibly positive difference in the bedroom, but it can also cause some unintended complications. Certain lubes will fail under specific circumstances, and some can even make sex less safe.

Here are the main types of lubricants and their best uses:

	Ideal for	Not ideal for	Noteworthy characteristics
Water based	Simple sex acts	Water or shower sex	Easy to clean up, may require frequent reapplication
Oil based	Water play, masturbation	Use with latex condoms	Degrades latex
Silicone based	Long sex sessions, water or shower sex	Use with silicone-based sex toys	Long-lasting, can be a pain to clean up

There are also hybrid lubricants available, many of which combine water-based and silicone-based materials.

If you use toys, keep them clean

For many people expanding their sexual repertoire, sex toys will enter the picture at some point. If you’re new to using sex toys, you may fail to clean them properly. That can result in a handful of unpleasant consequences, including dangerous infections. Fortunately, there are plenty of reputable sex toy cleaners on the market. Invest in one and use it regularly.

27 SOURCES

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2. Centers for Disease Control and Prevention. (2015). Sexual Risk Behaviors. Department of Health and Human Services. https://www.cdc.gov/youth-behavior/risk-behaviors/sexual-risk-behaviors.html
3. Herbenick, D., Patterson, C., Beckmeyer, J., Gonzalez, Y. R., Luetke, M., Valdivia, D. S., & Rosenberg, M. (2021). Diverse Sexual Behaviors in Undergraduate Students: Findings From a Campus Probability Survey. The Journal of Sexual Medicine, 18(6), 1024-1041.
4. Herbenick, D., Patterson, C., Fu, T., Gonzalez, Y.R.R., Luetke, M., Guerra-Reyes, L., Eastman-Mueller, H., Valdivia, D.S., Rosenberg, M. (2021, July 9). Prevalence and characteristics of choking/strangulation during sex: Findings from a probability survey of undergraduate students. Journal of American College Health 71(4), 1059–1073.
5. National Institute on Alcohol Abuse and Alcoholism. (2021, October). College Drinking. National Institutes of Health. Department of Health and Human Services. https://www.niaaa.nih.gov/publications/brochures-and-fact-sheets/college-drinking
6. Clark, N. (2017, May 10). The Etiology and Phenomenology of Sexual Shame: A Grounded Theory Study. Seattle Pacific University. https://digitalcommons.spu.edu/cgi/viewcontent.cgi?article=1024&context=cpy_etd
7. Hartford Healthcare. (2021, April 20). 1 in 4 College Students Has an STD: Here Are The Facts. https://healthnewshub.org/campus-care-1-in-4-college-students-has-an-std-here-are-the-facts/
8. Wooden, A. (2019, February 14) Why are we having less sex today than ever before? The Johns Hopkins News-Letter. https://www.jhunewsletter.com/article/2019/02/why-are-we-having-less-sex-today-than-ever-before
9. Johnson A., and Jackson, J. (2021, January 10). Sexually Transmitted Infections Among College Students. Microbiol Infect Dis. 2021; 5(1): 1-4.
10. World Health Organization. (2024, December 11). Herpes simplex virus. https://www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus
11. Cleveland Clinic. (2024, January 9). Mononucleosis. https://my.clevelandclinic.org/health/diseases/13974-mononucleosis
12. Centers for Disease Control and Prevention. (n.d.). Human Papillomavirus (HPV). https://www.cdc.gov/hpv/index.html
13. Mayo Clinic. (2024, September 12). Chlamydia trachomatis. https://www.mayoclinic.org/diseases-conditions/chlamydia/symptoms-causes/syc-20355349
14. Cleveland Clinic. (2024, July 25). Gonorrhea. https://my.clevelandclinic.org/health/diseases/4217-gonorrhea
15. Mayo Clinic. (2022, May 17). Trichomoniasis. https://www.mayoclinic.org/diseases-conditions/trichomoniasis/symptoms-causes/syc-20378609
16. Shen, J-H., Huang, K-Y.A., Chao-Yu, C., Chen, CJ., Lin, T-Y., Huang, Y-C. (2015, August 7). Seroprevalence of Herpes Simplex Virus Type 1 and 2 in Taiwan and Risk Factor Analysis, 2007. PLoS ONE 10(8): e0134178.
17. Cleveland Clinic. (2022, December 27). Syphilis. https://my.clevelandclinic.org/health/diseases/4622-syphilis
18. Centers for Disease Control and Prevention (n.d.). HIV. Department of Health and Human Services. https://www.cdc.gov/hiv/index.html
19. Planned Parenthood. (2022, April 1). STD vs STI — What’s the Difference? https://www.plannedparenthood.org/blog/std-vs-sti-whats-the-difference
20. Centers for Disease Control and Prevention. (2024, April 18). Types of Condoms. Department of Health and Human Services. https://www.cdc.gov/condom-use/communication-resources/?CDC_AAref_Val=https://www.cdc.gov/condomeffectiveness/brief.html
21. Richters, J., Prestage, G., Schneider, K., Clayton, S. (2010, June). Do women use dental dams? Safer sex practices of lesbians and other women who have sex with women. Sex Health. doi: 10.1071/SH09072.
22. Tosh, P.K. (2020, November 17). Mononucleosis and Epstein-Barr: What’s the connection? https://www.mayoclinic.org/diseases-conditions/mononucleosis/expert-answers/mononucleosis/faq-20058444
23. Oswalt, S.B., Eastman-Mueller, H.P., Baldwin, A.(2019, September 9). Counseling college students after a positive pregnancy test: Trends in practice. Journal of American College Health.
24. Substance Abuse and Mental Health Services Administration. (2023). National Survey on Drug Use and Health (NSDUH). https://www.samhsa.gov/data/data-we-collect/nsduh-national-survey-drug-use-and-health
25. Business Queensland. (2019, June 26). Signs that a person is unduly intoxicated. Queensland Government. https://www.business.qld.gov.au/industries/hospitality-tourism-sport/liquor-gaming/liquor/training/rsa/refresher/unduly-intoxicated/signs
26. Rape, Abuse, and Incest National Network. (n.d.). Campus Sexual Violence: Statistics. https://www.rainn.org/statistics/campus-sexual-violence
27. Renfro, K. J., Haderxhanaj, L., Coor, A., Eastman-Mueller, H., Oswalt, S., Kachur, R., Habel, M. A., Becasen, J. S., & Dittus, P. J. (2020). Sexual-risk and STI-testing behaviors of a national sample of non-students, two-year, and four-year college students. Journal of American College Health: J of ACH, 70(2), 544.

There shouldn’t be anything accompanying descriptions guide you in exploring the anatomy and physiology of the female reproductive organs. Learn all about fertilization, pregnancy, delivery, and lactation.

The female reproductive system includes the ovaries, fallopian tubes, uterus, vagina, vulva, mammary glands and breasts. These organs are involved in the production and transportation of gametes and the production of sex hormones. The female reproductive system also facilitates the fertilization of ova by sperm and supports the development of offspring during pregnancy and infancy.

Female Reproductive System Anatomy

Ovaries

The ovaries are a pair of small glands about the size and shape of almonds, located on the left and right sides of the pelvic body cavity lateral to the superior portion of the uterus. Ovaries produce female sex hormones such as estrogen and progesterone as well as ova (commonly called “eggs”), the female gametes. Ova are produced from oocyte cells that slowly develop throughout a woman’s early life and reach maturity after puberty. Each month during ovulation, a mature ovum is released. The ovum travels from the ovary to the fallopian tube, where it may be fertilized before reaching the uterus.

Fallopian Tubes

The fallopian tubes are a pair of muscular tubes that extend from the left and right superior corners of the uterus to the edge of the ovaries. The fallopian tubes end in a funnel-shaped structure called the infundibulum, which is covered with small finger-like projections called fimbriae. The fimbriae swipe over the outside of the ovaries to pick up released ova and carry them into the infundibulum for transport to the uterus. The inside of each fallopian tube is covered in cilia that work with the smooth muscle of the tube to carry the ovum to the uterus.

Uterus

The uterus is a hollow, muscular, pear-shaped organ located posterior and superior to the urinary bladder. Connected to the two fallopian tubes on its superior end and to the vagina (via the cervix) on its inferior end, the uterus is also known as the womb, as it surrounds and supports the developing fetus during pregnancy. The inner lining of the uterus, known as the endometrium, provides support to the embryo during early development. The visceral muscles of the uterus contract during childbirth to push the fetus through the birth canal.

Vagina

The vagina is an elastic, muscular tube that connects the cervix of the uterus to the exterior of the body. It is located inferior to the uterus and posterior to the urinary bladder. The vagina functions as the receptacle for the penis during sexual intercourse and carries sperm to the uterus and fallopian tubes. It also serves as the birth canal by stretching to allow delivery of the fetus during childbirth. During menstruation, the menstrual flow exits the body via the vagina.

Vulva

The vulva is the collective name for the external female genitalia located in the pubic region of the body. The vulva surrounds the external ends of the urethral opening and the vagina and includes the mons pubis, labia majora, labia minora, and clitoris. The mons pubis, or pubic mound, is a raised layer of adipose tissue between the skin and the pubic bone that provides cushioning to the vulva. The inferior portion of the mons pubis splits into left and right halves called the labia majora. The mons pubis and labia majora are covered with pubic hairs. Inside of the labia majora are smaller, hairless folds of skin called the labia minora that surround the vaginal and urethral openings. On the superior end of the labia minora is a small mass of erectile tissue known as the clitoris that contains many nerve endings for sensing sexual pleasure.

Breasts and Mammary Glands

The breasts are specialized organs of the female body that contain mammary glands, milk ducts, and adipose tissue. The two breasts are located on the left and right sides of the thoracic region of the body. In the center of each breast is a highly pigmented nipple that releases milk when stimulated. The areola, a thickened, highly pigmented band of skin that surrounds the nipple, protects the underlying tissues during breastfeeding. The mammary glands are a special type of sudoriferous glands that have been modified to produce milk to feed infants. Within each breast, 15 to 20 clusters of mammary glands become active during pregnancy and remain active until milk is no longer needed. The milk passes through milk ducts on its way to the nipple, where it exits the body.

Female Reproductive System Physiology

The Reproductive Cycle

The female reproductive cycle is the process of producing an ovum and readying the uterus to receive a fertilized ovum to begin pregnancy. If an ovum is produced but not fertilized and implanted in the uterine wall, the reproductive cycle resets itself through menstruation. The entire reproductive cycle takes about 28 days on average, but may be as short as 24 days or as long as 36 days for some women.

Oogenesis and Ovulation

Under the influence of follicle stimulating hormone (FSH), and luteinizing hormone (LH), the ovaries produce a mature ovum in a process known as ovulation. By about 14 days into the reproductive cycle, an oocyte reaches maturity and is released as an ovum. Although the ovaries begin to mature many oocytes each month, usually only one ovum per cycle is released.

Fertilization

Once the mature ovum is released from the ovary, the fimbriae catch the egg and direct it down the fallopian tube to the uterus. It takes about a week for the ovum to travel to the uterus. If sperm are able to reach and penetrate the ovum, the ovum becomes a fertilized zygote containing a full complement of DNA. After a two-week period of rapid cell division known as the germinal period of development, the zygote forms an embryo. The embryo will then implant itself into the uterine wall and develop there during pregnancy.

Menstruation

While the ovum matures and travels through the fallopian tube, the endometrium grows and develops in preparation for the embryo. If the ovum is not fertilized in time or if it fails to implant into the endometrium, the arteries of the uterus constrict to cut off blood flow to the endometrium. The lack of blood flow causes cell death in the endometrium and the eventual shedding of tissue in a process known as menstruation. In a normal menstrual cycle, this shedding begins around day 28 and continues into the first few days of the new reproductive cycle.

Pregnancy

If the ovum is fertilized by a sperm cell, the fertilized embryo will implant itself into the endometrium and begin to form an amniotic cavity, umbilical cord, and placenta. For the first 8 weeks, the embryo will develop almost all of the tissues and organs present in the adult before entering the fetal period of development during weeks 9 through 38. During the fetal period, the fetus grows larger and more complex until it is ready to be born.

Lactation

Lactation is the production and release of milk to feed an infant. The production of milk begins prior to birth under the control of the hormone prolactin. Prolactin is produced in response to the suckling of an infant on the nipple, so milk is produced as long as active breastfeeding occurs. As soon as an infant is weaned, prolactin and milk production end soon after. The release of milk by the nipples is known as the “milk-letdown reflex” and is controlled by the hormone oxytocin. Oxytocin is also produced in response to infant suckling so that milk is only released when an infant is actively feeding.

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys filter the blood to remove wastes and produce urine. The ureters, urinary bladder, and urethra together form the urinary tract, which acts as a plumbing system to drain urine from the kidneys, store it, and then release it during urination. Besides filtering and eliminating wastes from the body, the urinary system also maintains the homeostasis of water, ions, pH, blood pressure, calcium and red blood cells.

Urinary System Anatomy

Kidneys

The kidneys are a pair of bean-shaped organs found along the posterior wall of the abdominal cavity. The left kidney is located slightly higher than the right kidney because the right side of the liver is much larger than the left side. The kidneys, unlike the other organs of the abdominal cavity, are located posterior to the peritoneum and touch the muscles of the back. The kidneys are surrounded by a layer of adipose that holds them in place and protects them from physical damage. The kidneys filter metabolic wastes, excess ions, and chemicals from the blood to form urine.

Ureters

The ureters are a pair of tubes that carry urine from the kidneys to the urinary bladder. The ureters are about 10 to 12 inches long and run on the left and right sides of the body parallel to the vertebral column. Gravity and peristalsis of smooth muscle tissue in the walls of the ureters move urine toward the urinary bladder. The ends of the ureters extend slightly into the urinary bladder and are sealed at the point of entry to the bladder by the ureterovesical valves. These valves prevent urine from flowing back towards the kidneys.

Urinary Bladder

The urinary bladder is a sac-like hollow organ used for the storage of urine. The urinary bladder is located along the body’s midline at the inferior end of the pelvis. Urine entering the urinary bladder from the ureters slowly fills the hollow space of the bladder and stretches its elastic walls. The walls of the bladder allow it to stretch to hold anywhere from 600 to 800 milliliters of urine.

Urethra

The urethra is the tube through which urine passes from the bladder to the exterior of the body. The female urethra is around 2 inches long and ends inferior to the clitoris and superior to the vaginal opening. In males, the urethra is around 8 to 10 inches long and ends at the tip of the penis. The urethra is also an organ of the male reproductive system as it carries sperm out of the body through the penis.

The flow of urine through the urethra is controlled by the internal and external urethral sphincter muscles. The internal urethral sphincter is made of smooth muscle and opens involuntarily when the bladder reaches a certain set level of distention. The opening of the internal sphincter results in the sensation of needing to urinate. The external urethral sphincter is made of skeletal muscle and may be opened to allow urine to pass through the urethra or may be held closed to delay urination.

Urinary System Physiology

Maintenance of Homeostasis

The kidneys maintain the homeostasis of several important internal conditions by controlling the excretion of substances out of the body. 

Ions

The kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, and chloride ions into urine. In cases where these ions reach a higher than normal concentration, the kidneys can increase their excretion out of the body to return them to a normal level. Conversely, the kidneys can conserve these ions when they are present in lower than normal levels by allowing the ions to be reabsorbed into the blood during filtration. (See more about ions.)

pH

The kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism of dietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions into urine for elimination from the body. The kidneys also conserve bicarbonate ions, which act as important pH buffers in the blood.

Osmolarity

The cells of the body need to grow in an isotonic environment in order to maintain their fluid and electrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.

Blood Pressure

The kidneys monitor the body’s blood pressure to help maintain homeostasis. When blood pressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume of blood in the body. The kidneys are able to reduce blood volume by reducing the reabsorption of water into the blood and producing watery, dilute urine. When blood pressure becomes too low, the kidneys can produce the enzyme renin to constrict blood vessels and produce concentrated urine, which allows more water to remain in the blood.

Filtration

Inside each kidney are around a million tiny structures called nephrons. The nephron is the functional unit of the kidney that filters blood to produce urine. Arterioles in the kidneys deliver blood to a bundle of capillaries surrounded by a capsule called a glomerulus. As blood flows through the glomerulus, much of the blood’s plasma is pushed out of the capillaries and into the capsule, leaving the blood cells and a small amount of plasma to continue flowing through the capillaries. The liquid filtrate in the capsule flows through a series of tubules lined with filtering cells and surrounded by capillaries. The cells surrounding the tubules selectively absorb water and substances from the filtrate in the tubule and return it to the blood in the capillaries. At the same time, waste products present in the blood are secreted into the filtrate. By the end of this process, the filtrate in the tubule has become urine containing only water, waste products, and excess ions. The blood exiting the capillaries has reabsorbed all of the nutrients along with most of the water and ions that the body needs to function.

Storage and Excretion of Wastes

After urine has been produced by the kidneys, it is transported through the ureters to the urinary bladder. The urinary bladder fills with urine and stores it until the body is ready for its excretion. When the volume of the urinary bladder reaches anywhere from 150 to 400 milliliters, its walls begin to stretch and stretch receptors in its walls send signals to the brain and spinal cord. These signals result in the relaxation of the involuntary internal urethral sphincter and the sensation of needing to urinate. Urination may be delayed as long as the bladder does not exceed its maximum volume, but increasing nerve signals lead to greater discomfort and desire to urinate.

Urination is the process of releasing urine from the urinary bladder through the urethra and out of the body. The process of urination begins when the muscles of the urethral sphincters relax, allowing urine to pass through the urethra. At the same time that the sphincters relax, the smooth muscle in the walls of the urinary bladder contract to expel urine from the bladder.

Production of Hormones

The kidneys produce and interact with several hormones that are involved in the control of systems outside of the urinary system.

Calcitriol

Calcitriol is the active form of vitamin D in the human body. It is produced by the kidneys from precursor molecules produced by UV radiation striking the skin. Calcitriol works together with parathyroid hormone (PTH) to raise the level of calcium ions in the bloodstream. When the level of calcium ions in the blood drops below a threshold level, the parathyroid glands release PTH, which in turn stimulates the kidneys to release calcitriol. Calcitriol promotes the small intestine to absorb calcium from food and deposit it into the bloodstream. It also stimulates the osteoclasts of the skeletal system to break down bone matrix to release calcium ions into the blood.

Erythropoietin

Erythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate the production of red blood cells. The kidneys monitor the condition of the blood that passes through their capillaries, including the oxygen-carrying capacity of the blood. When the blood becomes hypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries begin producing EPO and release it into the bloodstream. EPO travels through the blood to the r**ed bone marrow**, where it stimulates hematopoietic cells to increase their rate of red blood cell production. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.

Renin

Renin is not a hormone itself, but an enzyme that the kidneys produce to start the renin-angiotensin system (RAS). The RAS increases blood volume and blood pressure in response to low blood pressure, blood loss, or dehydration. Renin is released into the blood where it catalyzes angiotensinogen from the liver into angiotensin I. Angiotensin I is further catalyzed by another enzyme into Angiotensin II.

Angiotensin II stimulates several processes, including stimulating the adrenal cortex to produce the hormone aldosterone. Aldosterone then changes the function of the kidneys to increase the reabsorption of water and sodium ions into the blood, increasing blood volume and raising blood pressure. Negative feedback from increased blood pressure finally turns off the RAS to maintain healthy blood pressure levels.

The immune and lymphatic systems are two closely related organ systems that share several organs and physiological functions. The immune system is our body’s defense system against infectious pathogenic viruses, bacteria, and fungi as well as parasitic animals and protists. The immune system works to keep these harmful agents out of the body and attacks those that manage to enter.

The lymphatic system is a system of capillaries, vessels, nodes and other organs that transport a fluid called lymph from the tissues as it returns to the bloodstream. The lymphatic tissue of these organs filters and cleans the lymph of any debris, abnormal cells, or pathogens. The lymphatic system also transports fatty acids from the intestines to the circulatory system.

Immune and Lymphatic System Anatomy

Red Bone Marrow and Leukocytes

Red bone marrow is a highly vascular tissue found in the spaces between trabeculae of spongy bone. It is mostly found in the ends of long bones and in the flat bones of the body. Red bone marrow is a hematopoietic tissue containing many stem cells that produce blood cells. All of the leukocytes, or white blood cells, of the immune system are produced by red bone marrow. Leukocytes can be further broken down into 2 groups based upon the type of stem cells that produces them: myeloid stem cells and lymphoid stem cells.  

Myeloid Stem Cells

Myeloid stem cells produce monocytes and the granular leukocytes—eosinophils, basophils, and neutrophils.

Monocytes are agranular leukocytes that can form 2 types of cells: macrophages and dendritic cells.

1. Macrophages. Monocytes respond slowly to infection and once present at the site of infection, develop into macrophages. Macrophages are phagocytes able to consume pathogens, destroyed cells, and debris by phagocytosis. As such, they have a role in both preventing infection as well as cleaning up the aftermath of an infection.
2. Dendritic cells. Monocytes also develop into dendritic cells in healthy tissues of the skin and mucous membranes. Dendritic cells are responsible for the detection of pathogenic antigens which are used to activate T cells and B cells.

Granular Leukocytes include the following:

1. Eosinophils. Eosinophils are granular leukocytes that reduce allergic inflammation and help the body fight off parasites.
2. Basophils. Basophils are granular leukocytes that trigger inflammation by releasing the chemicals heparin and histamine. Basophils are active in producing inflammation during allergic reactions and parasitic infections.
3. Neutrophils. Neutrophils are granular leukocytes that act as the first responders to the site of an infection. Neutrophils use chemotaxis to detect chemicals produced by infectious agents and quickly move to the site of infection. Once there, neutrophils ingest the pathogens via phagocytosis and release chemicals to trap and kill the pathogens.

Lymphoid Stem Cells

Lymphoid stem cells produce T lymphocytes and B lymphocytes.

• T lymphocytes. T lymphocytes, also commonly known as T cells, are cells involved in fighting specific pathogens in the body. T cells may act as helpers of other immune cells or attack pathogens directly. After an infection, memory T cells persist in the body to provide a faster reaction to subsequent infection by pathogens expressing the same antigen.
• B lymphocytes. B lymphocytes, also commonly known as B cells, are also cells involved in fighting specific pathogens in the body. Once B cells have been activated by contact with a pathogen, they form plasma cells that produce antibodies. Antibodies then neutralize the pathogens until other immune cells can destroy them. After an infection, memory B cells persist in the body to quickly produce antibodies to subsequent infection by pathogens expressing the same antigen.
• Natural killer cells. Natural killer cells, also known as NK cells, are lymphocytes that are able to respond to a wide range of pathogens and cancerous cells. NK cells travel within the blood and are found in the lymph nodes, spleen, and red bone marrow where they fight most types of infection.

Lymph Capillaries

As blood passes through the tissues of the body, it enters thin-walled capillaries to facilitate diffusion of nutrients, gases, and wastes. Blood plasma also diffuses through the thin capillary walls and penetrates into the spaces between the cells of the tissues. Some of this plasma diffuses back into the blood of the capillaries, but a considerable portion becomes embedded in the tissues as interstitial fluid. To prevent the accumulation of excess fluids, small dead-end vessels called lymphatic capillaries extend into the tissues to absorb fluids and return them to circulation.

Lymph

The interstitial fluid picked up by lymphatic capillaries is known as lymph. Lymph very closely resembles the plasma found in the veins: it is a mixture of about 90% water and 10% solutes such as proteins, cellular waste products, dissolved gases, and hormones. Lymph may also contain bacterial cells that are picked up from diseased tissues and the white blood cells that fight these pathogens. In late-stage cancer patients, lymph often contains cancerous cells that have metastasized from tumors and may form new tumors within the lymphatic system. A special type of lymph, known as chyle, is produced in the digestive system as lymph absorbs triglycerides from the intestinal villi. Due to the presence of triglycerides, chyle has a milky white coloration to it.

Lymphatic Vessels

Lymphatic capillaries merge together into larger lymphatic vessels to carry lymph through the body. The structure of lymphatic vessels closely resembles that of veins: they both have thin walls and many check valves due to their shared function of carrying fluids under low pressure. Lymph is transported through lymphatic vessels by the skeletal muscle pump—contractions of skeletal muscles constrict the vessels to push the fluid forward. Check valves prevent the fluid from flowing back toward the lymphatic capillaries.

Lymph Nodes

Lymph nodes are small, kidney-shaped organs of the lymphatic system. There are several hundred lymph nodes found mostly throughout the thorax and abdomen of the body with the highest concentrations in the axillary (armpit) and inguinal (groin) regions. The outside of each lymph node is made of a dense fibrous connective tissue capsule. Inside the capsule, the lymph node is filled with reticular tissue containing many lymphocytes and macrophages. The lymph nodes function as filters of lymph that enters from several afferent lymph vessels. The reticular fibers of the lymph node act as a net to catch any debris or cells that are present in the lymph. Macrophages and lymphocytes attack and kill any microbes caught in the reticular fibers. Efferent lymph vessels then carry the filtered lymph out of the lymph node and towards the lymphatic ducts.

Lymphatic Ducts

All of the lymphatic vessels of the body carry lymph toward the 2 lymphatic ducts: the thoracic duct and the right lymphatic ducts. These ducts serve to return lymph back to the venous blood supply so that it can be circulated as plasma.

• Thoracic duct. The thoracic duct connects the lymphatic vessels of the legs , abdomen, left arm, and the left side of the head, neck, and thorax to the left brachiocephalic vein.
• Right lymphatic duct. The right lymphatic duct connects the lymphatic vessels of the right arm and the right side of the head, neck, and thorax to the right brachiocephalic vein.

Lymphatic Nodules

Outside of the system of lymphatic vessels and lymph nodes, there are masses of non-encapsulated lymphatic tissue known as lymphatic nodules. The lymphatic nodules are associated with the mucous membranes of the body, where they work to protect the body from pathogens entering the body through open body cavities.

• Tonsils. There are 5 tonsils in the body—2 lingual, 2 palatine, and 1 pharyngeal. The lingual tonsils are located at the posterior root of the tongue near the pharynx. The palatine tonsils are in the posterior region of the mouth near the pharynx. The pharyngeal pharynx, also known as the adenoid, is found in the nasopharynx at the posterior end of the nasal cavity. The tonsils contain many T and B cells to protect the body from inhaled or ingested substances. The tonsils often become inflamed in response to an infection.
• Peyer’s patches. Peyer’s patches are small masses of lymphatic tissue found in the ileum of the small intestine. Peyer’s patches contain T and B cells that monitor the contents of the intestinal lumen for pathogens. Once the antigens of a pathogen are detected, the T and B cells spread and prepare the body to fight a possible infection.
• Spleen. The spleen is a flattened, oval-shaped organ located in the upper left quadrant of the abdomen lateral to the stomach. The spleen is made up of a dense fibrous connective tissue capsule filled with regions known as red and white pulp. Red pulp, which makes up most of the spleen’s mass, is so named because it contains many sinuses that filter the blood. Red pulp contains reticular tissues whose fibers filter worn out or damaged red blood cells from the blood. Macrophages in the red pulp digest and recycle the hemoglobin of the captured red blood cells. The red pulp also stores many platelets to be released in response to blood loss. White pulp is found within the red pulp surrounding the arterioles of the spleen. It is made of lymphatic tissue and contains many T cells, B cells, and macrophages to fight off infections.
• Thymus. The thymus is a small, triangular organ found just posterior to the sternum and anterior to the heart. The thymus is mostly made of glandular epithelium and hematopoietic connective tissues. The thymus produces and trains T cells during fetal development and childhood. T cells formed in the thymus and red bone marrow mature, develop, and reproduce in the thymus throughout childhood. The vast majority of T cells do not survive their training in the thymus and are destroyed by macrophages. The surviving T cells spread throughout the body to the other lymphatic tissues to fight infections. By the time a person reaches puberty, the immune system is mature and the role of the thymus is diminished. After puberty, the inactive thymus is slowly replaced by adipose tissue.

Immune and Lymphatic System Physiology

Lymph Circulation

One of the primary functions of the lymphatic system is the movement of interstitial fluid from the tissues to the circulatory system. Like the veins of the circulatory system, lymphatic capillaries and vessels move lymph with very little pressure to help with circulation. To help move lymph towards the lymphatic ducts, there is a series of many one-way check valves found throughout the lymphatic vessels. These check valves allow lymph to move toward the lymphatic ducts and close when lymph attempts to flow away from the ducts. In the limbs, skeletal muscle contraction squeezes the walls of lymphatic vessels to push lymph through the valves and towards the thorax. In the trunk, the diaphragm pushes down into the abdomen during inhalation. This increased abdominal pressure pushes lymph into the less pressurized thorax. The pressure gradient reverses during exhalation, but the check valves prevent lymph from being pushed backwards.

Transport of Fatty Acids

Another major function of the lymphatic system is the transportation of fatty acids from the digestive system. The digestive system breaks large macromolecules of carbohydrates, proteins, and lipids into smaller nutrients that can be absorbed through the villi of the intestinal wall. Most of these nutrients are absorbed directly into the bloodstream, but most fatty acids, the building blocks of fats, are absorbed through the lymphatic system.

In the villi of the small intestine are lymphatic capillaries called lacteals. Lacteals are able to absorb fatty acids from the intestinal epithelium and transport them along with lymph. The fatty acids turn the lymph into a white, milky substance called chyle. Chyle is transported through lymphatic vessels to the thoracic duct where it enters the bloodstream and travels to the liver to be metabolized.

Types of Immunity

The body employs many different types of immunity to protect itself from infection from a seemingly endless supply of pathogens. These defenses may be external and prevent pathogens from entering the body. Conversely, internal defenses fight pathogens that have already entered the body. Among the internal defenses, some are specific to only one pathogen or may be innate and defend against many pathogens. Some of these specific defenses can be acquired to preemptively prevent an infection before a pathogen enters the body.

The body has many innate ways to defend itself against a broad spectrum of pathogens. These defenses may be external or internal defenses.

External defenses include the following:

• The coverings and linings of the body constantly prevent infections before they begin by barring pathogens from entering the body. Epidermal cells are constantly growing, dying, and shedding to provide a renewed physical barrier to pathogens.
• Secretions like sebum, cerumen, mucus, tears, and saliva are used to trap, move, and sometimes even kill bacteria that settle on or in the body. Stomach acid acts as a chemical barrier to kill microbes found on food entering the body. Urine and acidic vaginal secretions also help to kill and remove pathogens that attempt to enter the body.
• The flora of naturally occurring beneficial bacteria that live on and in our bodies provide a layer of protection from harmful microbes that would seek to colonize our bodies for themselves.

Internal defenses include fever, inflammation, natural killer cells, and phagocytes. Let’s explore internal defenses in greater detail.

Fever

In response to an infection, the body may start a fever by raising its internal temperature out of its normal homeostatic range. Fevers help to speed up the body’s response system to an infection while at the same time slowing the reproduction of the pathogen.

Inflammation

The body may also start an inflammation in a region of the body to stop the spread of the infection. Inflammations are the result of a localized vasodilation that allows extra blood to flow into the infected region. The extra blood flow speeds the arrival of leukocytes to fight the infection. The enlarged blood vessel allows fluid and cells to leak out of the blood vessel to cause swelling and the movement of leukocytes into the tissue to fight the infection.

Natural Killer Cells

Natural killer (NK) cells are special lymphocytes that are able to recognize and kill virus-infected cells and tumor cells. NK cells check the surface markers on the surface of the body’s cells, looking for cells that are lacking the correct number of markers due to disease. The NK cells then kill these cells before they can spread infection or cancer.

Phagocytes

The term phagocyte means “eating cell” and refers to a group of cell types including neutrophils and macrophages. A phagocyte engulfs pathogens with its cell membrane before using digestive enzymes to kill and dissolve the cell into its chemical parts. Phagocytes are able to recognize and consume many different types of cells, including dead or damaged body cells.

Cell-mediated Specific Immunity

When a pathogen infects the body, it often encounters macrophages and dendritic cells of the innate immune system. These cells can become antigen-presenting cells (APCs) by consuming and processing pathogenic antigens. The APCs travel into the lymphatic system carrying these antigens to be presented to the T cells and B cells of the specific immune system.

Inactive T cells are found in lymphatic tissue awaiting infection by a pathogen. Certain T cells have antigen receptors that recognize the pathogen but do not reproduce until they are triggered by an APC. The activated T cell begins reproducing very quickly to form an army of active T cells that spread through the body and fight the pathogen. Cytotoxic T cells directly attach to and kill pathogens and virus-infected cells using powerful toxins. Helper T cells assist in the immune response by stimulating the response of B cells and macrophages.

After an infection has been fought off, memory T cells remain in the lymphatic tissue waiting for a new infection by cells presenting the same antigen. The response by memory T cells to the antigen is much faster than that of the inactive T cells that fought the first infection. The increase in T cell reaction speed leads to immunity—the reintroduction of the same pathogen is fought off so quickly that there are few or no symptoms. This immunity may last for years or even an entire lifetime.

Antibody-mediated Specific Immunity

During an infection, the APCs that travel to the lymphatic system to stimulate T cells also stimulate B cells. B cells are lymphocytes that are found in lymphatic tissues of the body that produce antibodies to fight pathogens (instead of traveling through the body themselves). Once a B cell has been contacted by an APC, it processes the antigen to produce an MHC-antigen complex. Helper T cells present in the lymphatic system bind to the MHC-antigen complex to stimulate the B cell to become active. The active B cell begins to reproduce and produce 2 types of cells: plasma cells and memory B cells.

1. Plasma cells become antibody factories producing thousands of antibodies.
2. Memory B cells reside in the lymphatic system where they help to provide immunity by preparing for later infection by the same antigen-presenting pathogen.

Antibodies are proteins that are specific to and bind to a particular antigen on a cell or virus. Once antibodies have latched on to a cell or virus, they make it harder for their target to move, reproduce, and infect cells. Antibodies also make it easier and more appealing for phagocytes to consume the pathogen.

Acquired Immunity

Under most circumstances, immunity is developed throughout a lifetime by the accumulation of memory T and B cells after an infection. There are a few ways that immunity can be acquired without exposure to a pathogen. Immunization is the process of introducing antigens from a virus or bacterium to the body so that memory T and B cells are produced to prevent an actual infection. Most immunizations involve the injection of bacteria or viruses that have been inactivated or weakened. Newborn infants can also acquire some temporary immunity from infection thanks to antibodies that are passed on from their mother. Some antibodies are able to cross the placenta from the mother’s blood and enter the infant’s bloodstream. Other antibodies are passed through breast milk to protect the infant.

The cells of the human body require a constant stream of oxygen to stay alive. The respiratory system provides oxygen to the body’s cells while removing carbon dioxide, a waste product that can be lethal if allowed to accumulate. There are 3 major parts of the respiratory system: the airway, the lungs, and the muscles of respiration. The airway, which includes the nose, mouth, pharynx, larynx, trachea, bronchi, and bronchioles, carries air between the lungs and the body’s exterior. The lungs act as the functional units of the respiratory system by passing oxygen into the body and carbon dioxide out of the body. Finally, the muscles of respiration, including the diaphragm and intercostal muscles, work together to act as a pump, pushing air into and out of the lungs during breathing.

Anatomy of the Respiratory System

Nose and Nasal Cavity

The nose and nasal cavity form the main external opening for the respiratory system and are the first section of the body’s airway—the respiratory tract through which air moves. The nose is a structure of the face made of cartilage, bone, muscle, and skin that supports and protects the anterior portion of the nasal cavity. The nasal cavity is a hollow space within the nose and skull that is lined with hairs and mucus membrane. The function of the nasal cavity is to warm, moisturize, and filter air entering the body before it reaches the lungs. Hairs and mucus lining the nasal cavity help to trap dust, mold, pollen and other environmental contaminants before they can reach the inner portions of the body. Air exiting the body through the nose returns moisture and heat to the nasal cavity before being exhaled into the environment.

Mouth

The mouth, also known as the oral cavity, is the secondary external opening for the respiratory tract. Most normal breathing takes place through the nasal cavity, but the oral cavity can be used to supplement or replace the nasal cavity’s functions when needed. Because the pathway of air entering the body from the mouth is shorter than the pathway for air entering from the nose, the mouth does not warm and moisturize the air entering the lungs as well as the nose performs this function. The mouth also lacks the hairs and sticky mucus that filter air passing through the nasal cavity. The one advantage of breathing through the mouth is that its shorter distance and larger diameter allows more air to quickly enter the body.

Pharynx

The pharynx, also known as the throat, is a muscular funnel that extends from the posterior end of the nasal cavity to the superior end of the esophagus and larynx. The pharynx is divided into 3 regions: the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx is the superior region of the pharynx found in the posterior of the nasal cavity. Inhaled air from the nasal cavity passes into the nasopharynx and descends through the oropharynx, located in the posterior of the oral cavity. Air inhaled through the oral cavity enters the pharynx at the oropharynx. The inhaled air then descends into the laryngopharynx, where it is diverted into the opening of the larynx by the epiglottis. The epiglottis is a flap of elastic cartilage that acts as a switch between the trachea and the esophagus. Because the pharynx is also used to swallow food, the epiglottis ensures that air passes into the trachea by covering the opening to the esophagus. During the process of swallowing, the epiglottis moves to cover the trachea to ensure that food enters the esophagus and to prevent choking.

Larynx

The larynx, also known as the voice box, is a short section of the airway that connects the laryngopharynx and the trachea. The larynx is located in the anterior portion of the neck, just inferior to the hyoid bone and superior to the trachea. Several cartilage structures make up the larynx and give it its structure. The epiglottis is one of the cartilage pieces of the larynx and serves as the cover of the larynx during swallowing. Inferior to the epiglottis is the thyroid cartilage, which is often referred to as the Adam’s apple as it is most commonly enlarged and visible in adult males. The thyroid holds open the anterior end of the larynx and protects the vocal folds. Inferior to the thyroid cartilage is the ring-shaped cricoid cartilage which holds the larynx open and supports its posterior end. In addition to cartilage, the larynx contains special structures known as vocal folds, which allow the body to produce the sounds of speech and singing. The vocal folds are folds of mucous membrane that vibrate to produce vocal sounds. The tension and vibration speed of the vocal folds can be changed to change the pitch that they produce.

Trachea

The trachea, or windpipe, is a 5-inch long tube made of C-shaped hyaline cartilage rings lined with pseudostratified ciliated columnar epithelium. The trachea connects the larynx to the bronchi and allows air to pass through the neck and into the thorax. The rings of cartilage making up the trachea allow it to remain open to air at all times. The open end of the cartilage rings faces posteriorly toward the esophagus, allowing the esophagus to expand into the space occupied by the trachea to accommodate masses of food moving through the esophagus.

The main function of the trachea is to provide a clear airway for air to enter and exit the lungs. In addition, the epithelium lining the trachea produces mucus that traps dust and other contaminants and prevents it from reaching the lungs. Cilia on the surface of the epithelial cells move the mucus superiorly toward the pharynx where it can be swallowed and digested in the gastrointestinal tract.

Bronchi and Bronchioles

At the inferior end of the trachea, the airway splits into left and right branches known as the primary bronchi. The left and right bronchi run into each lung before branching off into smaller secondary bronchi. The secondary bronchi carry air into the lobes of the lungs—2 in the left lung and 3 in the right lung. The secondary bronchi in turn split into many smaller tertiary bronchi within each lobe. The **tertiary bronchi **split into many smaller bronchioles that spread throughout the lungs. Each bronchiole further splits into many smaller branches less than a millimeter in diameter called terminal bronchioles. Finally, the millions of tiny terminal bronchioles conduct air to the alveoli of the lungs.

As the airway splits into the tree-like branches of the bronchi and bronchioles, the structure of the walls of the airway begins to change. The primary bronchi contain many C-shaped cartilage rings that firmly hold the airway open and give the bronchi a cross-sectional shape like a flattened circle or a letter D. As the bronchi branch into secondary and tertiary bronchi, the cartilage becomes more widely spaced and more smooth muscle and elastin protein is found in the walls. The bronchioles differ from the structure of the bronchi in that they do not contain any cartilage at all. The presence of smooth muscles and elastin allow the smaller bronchi and bronchioles to be more flexible and contractile.

The main function of the bronchi and bronchioles is to carry air from the trachea into the lungs. Smooth muscle tissue in their walls helps to regulate airflow into the lungs. When greater volumes of air are required by the body, such as during exercise, the smooth muscle relaxes to dilate the bronchi and bronchioles. The dilated airway provides less resistance to airflow and allows more air to pass into and out of the lungs. The smooth muscle fibers are able to contract during rest to prevent hyperventilation. The bronchi and bronchioles also use the mucus and cilia of their epithelial lining to trap and move dust and other contaminants away from the lungs.

Lungs

The lungs are a pair of large, spongy organs found in the thorax lateral to the heart and superior to the diaphragm. Each lung is surrounded by a pleural membrane that provides the lung with space to expand as well as a negative pressure space relative to the body’s exterior. The negative pressure allows the lungs to passively fill with air as they relax. The left and right lungs are slightly different in size and shape due to the heart pointing to the left side of the body. The left lung is therefore slightly smaller than the right lung and is made up of 2 lobes while the right lung has 3 lobes.

The interior of the lungs is made up of spongy tissues containing many capillaries and around 30 million tiny sacs known as alveoli. The alveoli are cup-shaped structures found at the end of the terminal bronchioles and surrounded by capillaries. The alveoli are lined with thin simple squamous epithelium that allows air entering the alveoli to exchange its gases with the blood passing through the capillaries.

Muscles of Respiration

Surrounding the lungs are sets of muscles that are able to cause air to be inhaled or exhaled from the lungs. The principal muscle of respiration in the human body is the diaphragm, a thin sheet of skeletal muscle that forms the floor of the thorax. When the diaphragm contracts, it moves inferiorly a few inches into the abdominal cavity, expanding the space within the thoracic cavity and pulling air into the lungs. Relaxation of the diaphragm allows air to flow back out the lungs during exhalation.

Between the ribs are many small intercostal muscles that assist the diaphragm with expanding and compressing the lungs. These muscles are divided into 2 groups: the internal intercostal muscles and the external intercostal muscles. The internal intercostal muscles are the deeper set of muscles and depress the ribs to compress the thoracic cavity and force air to be exhaled from the lungs. The external intercostals are found superficial to the internal intercostals and function to elevate the ribs, expanding the volume of the thoracic cavity and causing air to be inhaled into the lungs.

Physiology of the Respiratory System

Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs to facilitate gas exchange. The respiratory system uses both a negative pressure system and the contraction of muscles to achieve pulmonary ventilation. The negative pressure system of the respiratory system involves the establishment of a negative pressure gradient between the alveoli and the external atmosphere. The pleural membrane seals the lungs and maintains the lungs at a pressure slightly below that of the atmosphere when the lungs are at rest. This results in air following the pressure gradient and passively filling the lungs at rest. As the lungs fill with air, the pressure within the lungs rises until it matches the atmospheric pressure. At this point, more air can be inhaled by the contraction of the diaphragm and the external intercostal muscles, increasing the volume of the thorax and reducing the pressure of the lungs below that of the atmosphere again.

To exhale air, the diaphragm and external intercostal muscles relax while the internal intercostal muscles contract to reduce the volume of the thorax and increase the pressure within the thoracic cavity. The pressure gradient is now reversed, resulting in the exhalation of air until the pressures inside the lungs and outside of the body are equal. At this point, the elastic nature of the lungs causes them to recoil back to their resting volume, restoring the negative pressure gradient present during inhalation.

External Respiration

External respiration is the exchange of gases between the air filling the alveoli and the blood in the capillaries surrounding the walls of the alveoli. Air entering the lungs from the atmosphere has a higher partial pressure of oxygen and a lower partial pressure of carbon dioxide than does the blood in the capillaries. The difference in partial pressures causes the gases to diffuse passively along their pressure gradients from high to low pressure through the simple squamous epithelium lining of the alveoli. The net result of external respiration is the movement of oxygen from the air into the blood and the movement of carbon dioxide from the blood into the air. The oxygen can then be transported to the body’s tissues while carbon dioxide is released into the atmosphere during exhalation.

Internal Respiration

Internal respiration is the exchange of gases between the blood in capillaries and the tissues of the body. Capillary blood has a higher partial pressure of oxygen and a lower partial pressure of carbon dioxide than the tissues through which it passes. The difference in partial pressures leads to the diffusion of gases along their pressure gradients from high to low pressure through the endothelium lining of the capillaries. The net result of internal respiration is the diffusion of oxygen into the tissues and the diffusion of carbon dioxide into the blood.

Transportation of Gases

The 2 major respiratory gases, oxygen and carbon dioxide, are transported through the body in the blood. Blood plasma has the ability to transport some dissolved oxygen and carbon dioxide, but most of the gases transported in the blood are bonded to transport molecules. Hemoglobin is an important transport molecule found in red blood cells that carries almost 99% of the oxygen in the blood. Hemoglobin can also carry a small amount of carbon dioxide from the tissues back to the lungs. However, the vast majority of carbon dioxide is carried in the plasma as bicarbonate ion. When the partial pressure of carbon dioxide is high in the tissues, the enzyme carbonic anhydrase catalyzes a reaction between carbon dioxide and water to form carbonic acid. Carbonic acid then dissociates into hydrogen ion and bicarbonate ion. When the partial pressure of carbon dioxide is low in the lungs, the reactions reverse and carbon dioxide is liberated into the lungs to be exhaled.

Homeostatic Control of Respiration

Under normal resting conditions, the body maintains a quiet breathing rate and depth called eupnea. Eupnea is maintained until the body’s demand for oxygen and production of carbon dioxide rises due to greater exertion. Autonomic chemoreceptors in the body monitor the partial pressures of oxygen and carbon dioxide in the blood and send signals to the respiratory center of the brain stem. The respiratory center then adjusts the rate and depth of breathing to return the blood to its normal levels of gas partial pressures.

Health Issues Affecting the Respiratory System

When something impairs our ability to exchange carbon dioxide for oxygen, this is obviously a serious problem. Many health problems can cause respiratory problems, from allergies and asthma to pneumonia and lung cancer. The causes of these issues are just as varied—among them, infection (bacterial or viral), environmental exposure (pollution or cigarette smoke, for instance), genetic inheritance or a combination of factors. Sometimes the onset is so gradual, we don’t seek medical attention until the condition has advanced. Sometimes, as with the genetic disorder called alpha-1 antitrypsin deficiency (A1AD), symptoms gradually set in and are often under-diagnosed or misdiagnosed. DNA health testing can screen you for genetic risk of A1AD.

The nervous system consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts. The brain and spinal cord form the control center known as the central nervous system (CNS), where information is evaluated and decisions made. The sensory nerves and sense organs of the peripheral nervous system (PNS) monitor conditions inside and outside of the body and send this information to the CNS. Efferent nerves in the PNS carry signals from the control center to the muscles, glands, and organs to regulate their functions.

Nervous System Anatomy

Nervous Tissue

The majority of the nervous system is tissue made up of two classes of cells: neurons and neuroglia.

Neurons

Neurons, also known as nerve cells, communicate within the body by transmitting electrochemical signals. Neurons look quite different from other cells in the body due to the many long cellular processes that extend from their central cell body. The cell body is the roughly round part of a neuron that contains the nucleus, mitochondria, and most of the cellular organelles. Small tree-like structures called dendrites extend from the cell body to pick up stimuli from the environment, other neurons, or sensory receptor cells. Long transmitting processes called axons extend from the cell body to send signals onward to other neurons or effector cells in the body. 

There are 3 basic classes of neurons: afferent neurons, efferent neurons, and interneurons.

1. Afferent neurons. Also known as sensory neurons, afferent neurons transmit sensory signals to the central nervous system from receptors in the body.
2. Efferent neurons. Also known as motor neurons, efferent neurons transmit signals from the central nervous system to effectors in the body such as muscles and glands.
3. Interneurons. Interneurons form complex networks within the central nervous system to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons.

Neuroglia

Neuroglia, also known as glial cells, act as the “helper” cells of the nervous system. Each neuron in the body is surrounded by anywhere from 6 to 60 neuroglia that protect, feed, and insulate the neuron. Because neurons are extremely specialized cells that are essential to body function and almost never reproduce, neuroglia are vital to maintaining a functional nervous system.

Brain

The brain, a soft, wrinkled organ that weighs about 3 pounds, is located inside the cranial cavity, where the bones of the skull surround and protect it. The approximately 100 billion neurons of the brain form the main control center of the body. The brain and spinal cord together form the central nervous system (CNS), where information is processed and responses originate. The brain, the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions, also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion.

Spinal Cord

The spinal cord is a long, thin mass of bundled neurons that carries information through the vertebral cavity of the spine beginning at the medulla oblongata of the brain on its superior end and continuing inferiorly to the lumbar region of the spine. In the lumbar region, the spinal cord separates into a bundle of individual nerves called the cauda equina (due to its resemblance to a horse’s tail) that continues inferiorly to the sacrum and coccyx. The white matter of the spinal cord functions as the main conduit of nerve signals to the body from the brain. The grey matter of the spinal cord integrates reflexes to stimuli.

Nerves

Nerves are bundles of axons in the peripheral nervous system (PNS) that act as information highways to carry signals between the brain and spinal cord and the rest of the body. Each axon is wrapped in a connective tissue sheath called the endoneurium. Individual axons of the nerve are bundled into groups of axons called fascicles, wrapped in a sheath of connective tissue called the perineurium. Finally, many fascicles are wrapped together in another layer of connective tissue called the epineurium to form a whole nerve. The wrapping of nerves with connective tissue helps to protect the axons and to increase the speed of their communication within the body.

• Afferent, Efferent, and Mixed Nerves. Some of the nerves in the body are specialized for carrying information in only one direction, similar to a one-way street. Nerves that carry information from sensory receptors to the central nervous system only are called afferent nerves. Other neurons, known as efferent nerves, carry signals only from the central nervous system to effectors such as muscles and glands. Finally, some nerves are mixed nerves that contain both afferent and efferent axons. Mixed nerves function like 2-way streets where afferent axons act as lanes heading toward the central nervous system and efferent axons act as lanes heading away from the central nervous system.
• Cranial Nerves. Extending from the inferior side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is identified by a Roman numeral 1 to 12 based upon its location along the anterior-posterior axis of the brain. Each nerve also has a descriptive name (e.g. olfactory, optic, etc.) that identifies its function or location. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the head, neck, and shoulders, the heart, and the GI tract.
• Spinal Nerves. Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that carry both sensory and motor signals between the spinal cord and specific regions of the body. The 31 spinal nerves are split into 5 groups named for the 5 regions of the vertebral column. Thus, there are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Each spinal nerve exits from the spinal cord through the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull.

Meninges

The meninges are the protective coverings of the central nervous system (CNS). They consist of three layers: the dura mater, arachnoid mater, and pia mater.

• Dura mater. The dura mater, which means “tough mother,” is the thickest, toughest, and most superficial layer of meninges. Made of dense irregular connective tissue, it contains many tough collagen fibers and blood vessels. Dura mater protects the CNS from external damage, contains the cerebrospinal fluid that surrounds the CNS, and provides blood to the nervous tissue of the CNS.
• Arachnoid mater. The arachnoid mater, which means “spider-like mother,” is much thinner and more delicate than the dura mater. It lines the inside of the dura mater and contains many thin fibers that connect it to the underlying pia mater. These fibers cross a fluid-filled space called the subarachnoid space between the arachnoid mater and the pia mater.
• Pia mater. The pia mater, which means “tender mother,” is a thin and delicate layer of tissue that rests on the outside of the brain and spinal cord. Containing many blood vessels that feed the nervous tissue of the CNS, the pia mater penetrates into the valleys of the sulci and fissures of the brain as it covers the entire surface of the CNS.

Cerebrospinal Fluid

The space surrounding the organs of the CNS is filled with a clear fluid known as cerebrospinal fluid (CSF). CSF is formed from blood plasma by special structures called choroid plexuses. The choroid plexuses contain many capillaries lined with epithelial tissue that filters blood plasma and allows the filtered fluid to enter the space around the brain.  

Newly created CSF flows through the inside of the brain in hollow spaces called ventricles and through a small cavity in the middle of the spinal cord called the central canal. CSF also flows through the subarachnoid space around the outside of the brain and spinal cord. CSF is constantly produced at the choroid plexuses and is reabsorbed into the bloodstream at structures called arachnoid villi.

Cerebrospinal fluid provides several vital functions to the central nervous system:

1. CSF absorbs shocks between the brain and skull and between the spinal cord and vertebrae. This shock absorption protects the CNS from blows or sudden changes in velocity, such as during a car accident.
2. The brain and spinal cord float within the CSF, reducing their apparent weight through buoyancy. The brain is a very large but soft organ that requires a high volume of blood to function effectively. The reduced weight in cerebrospinal fluid allows the blood vessels of the brain to remain open and helps protect the nervous tissue from becoming crushed under its own weight.
3. CSF helps to maintain chemical homeostasis within the central nervous system. It contains ions, nutrients, oxygen, and albumins that support the chemical and osmotic balance of nervous tissue. CSF also removes waste products that form as byproducts of cellular metabolism within nervous tissue.

Sense Organs

All of the bodies’ many sense organs are components of the nervous system. What are known as the special senses—vision, taste, smell, hearing, and balance—are all detected by specialized organs such as the eyes, taste buds, and olfactory epithelium. Sensory receptors for the general senses like touch, temperature, and pain are found throughout most of the body. All of the sensory receptors of the body are connected to afferent neurons that carry their sensory information to the CNS to be processed and integrated.

Nervous System Physiology

Functions of the Nervous System

The nervous system has 3 main functions: sensory, integration, and motor.   

1. Sensory. The sensory function of the nervous system involves collecting information from sensory receptors that monitor the body’s internal and external conditions. These signals are then passed on to the central nervous system (CNS) for further processing by afferent neurons (and nerves).
2. Integration. The process of integration is the processing of the many sensory signals that are passed into the CNS at any given time. These signals are evaluated, compared, used for decision making, discarded or committed to memory as deemed appropriate. Integration takes place in the gray matter of the brain and spinal cord and is performed by interneurons. Many interneurons work together to form complex networks that provide this processing power.
3. Motor. Once the networks of interneurons in the CNS evaluate sensory information and decide on an action, they stimulate efferent neurons. Efferent neurons (also called motor neurons) carry signals from the gray matter of the CNS through the nerves of the peripheral nervous system to effector cells. The effector may be smooth, cardiac, or skeletal muscle tissue or glandular tissue. The effector then releases a hormone or moves a part of the body to respond to the stimulus.

Unfortunately of course, our nervous system doesn’t always function as it should. Sometimes this is the result of diseases like Alzheimer’s and Parkinson’s disease. Did you know that DNA testing can help you discover your genetic risk of acquiring certain health conditions that affect the organs of our nervous system? Late-onset Alzheimer’s, Parkinson’s disease, macular degeneration – visit our guide to DNA health testing to find out more.

Divisions of the Nervous System

Central Nervous System

The brain and spinal cord together form the central nervous system, or CNS. The CNS acts as the control center of the body by providing its processing, memory, and regulation systems. The CNS takes in all of the conscious and subconscious sensory information from the body’s sensory receptors to stay aware of the body’s internal and external conditions. Using this sensory information, it makes decisions about both conscious and subconscious actions to take to maintain the body’s homeostasis and ensure its survival. The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality. The brain is the seat of consciousness and determines who we are as individuals.

Peripheral Nervous System

The peripheral nervous system (PNS) includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors.

Somatic Nervous System

The somatic nervous system (SNS) is a division of the PNS that includes all of the voluntary efferent neurons. The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body.

Autonomic Nervous System

The autonomic nervous system (ANS) is a division of the PNS that includes all of the involuntary efferent neurons. The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue.

There are 2 divisions of the autonomic nervous system in the body: the sympathetic and parasympathetic divisions.

• Sympathetic. The sympathetic division forms the body’s “fight or flight” response to stress, danger, excitement, exercise, emotions, and embarrassment. The sympathetic division increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations.
• Parasympathetic. The parasympathetic division forms the body’s “rest and digest” response when the body is relaxed, resting, or feeding. The parasympathetic works to undo the work of the sympathetic division after a stressful situation. Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes.

Enteric Nervous System

The enteric nervous system (ENS) is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs. The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions. However, the ENS mostly works independently of the CNS and continues to function without any outside input. For this reason, the ENS is often called the “brain of the gut” or the body’s “second brain.” The ENS is an immense system—almost as many neurons exist in the ENS as in the spinal cord.

Action Potentials

Neurons function through the generation and propagation of electrochemical signals known as action potentials (APs). An AP is created by the movement of sodium and potassium ions through the membrane of neurons. (See Water and Electrolytes.)

• Resting Potential. At rest, neurons maintain a concentration of sodium ions outside of the cell and potassium ions inside of the cell. This concentration is maintained by the sodium-potassium pump of the cell membrane which pumps 3 sodium ions out of the cell for every 2 potassium ions that are pumped into the cell. The ion concentration results in a resting electrical potential of -70 millivolts (mV), which means that the inside of the cell has a negative charge compared to its surroundings.
• Threshold Potential. If a stimulus permits enough positive ions to enter a region of the cell to cause it to reach -55 mV, that region of the cell will open its voltage-gated sodium channels and allow sodium ions to diffuse into the cell. -55 mV is the threshold potential for neurons as this is the “trigger” voltage that they must reach to cross the threshold into forming an action potential.
• Depolarization. Sodium carries a positive charge that causes the cell to become depolarized (positively charged) compared to its normal negative charge. The voltage for depolarization of all neurons is +30 mV. The depolarization of the cell is the AP that is transmitted by the neuron as a nerve signal. The positive ions spread into neighboring regions of the cell, initiating a new AP in those regions as they reach -55 mV. The AP continues to spread down the cell membrane of the neuron until it reaches the end of an axon.
• Repolarization. After the depolarization voltage of +30 mV is reached, voltage-gated potassium ion channels open, allowing positive potassium ions to diffuse out of the cell. The loss of potassium along with the pumping of sodium ions back out of the cell through the sodium-potassium pump restores the cell to the -55 mV resting potential. At this point the neuron is ready to start a new action potential.

Synapses

A synapse is the junction between a neuron and another cell. Synapses may form between 2 neurons or between a neuron and an effector cell. There are two types of synapses found in the body: chemical synapses and electrical synapses.

• Chemical synapses. At the end of a neuron’s axon is an enlarged region of the axon known as the axon terminal. The axon terminal is separated from the next cell by a small gap known as the synaptic cleft. When an AP reaches the axon terminal, it opens voltage-gated calcium ion channels. Calcium ions cause vesicles containing chemicals known as neurotransmitters (NT) to release their contents by exocytosis into the synaptic cleft. The NT molecules cross the synaptic cleft and bind to receptor molecules on the cell, forming a synapse with the neuron. These receptor molecules open ion channels that may either stimulate the receptor cell to form a new action potential or may inhibit the cell from forming an action potential when stimulated by another neuron.
• Electrical synapses. Electrical synapses are formed when 2 neurons are connected by small holes called gap junctions. The gap junctions allow electric current to pass from one neuron to the other, so that an AP in one cell is passed directly on to the other cell through the synapse.

Myelination

The axons of many neurons are covered by a coating of insulation known as myelin to increase the speed of nerve conduction throughout the body. Myelin is formed by 2 types of glial cells: Schwann cells in the PNS and oligodendrocytes in the CNS. In both cases, the glial cells wrap their plasma membrane around the axon many times to form a thick covering of lipids. The development of these myelin sheaths is known as myelination.

Myelination speeds up the movement of APs in the axon by reducing the number of APs that must form for a signal to reach the end of an axon. The myelination process begins speeding up nerve conduction in fetal development and continues into early adulthood. Myelinated axons appear white due to the presence of lipids and form the white matter of the inner brain and outer spinal cord. White matter is specialized for carrying information quickly through the brain and spinal cord. The gray matter of the brain and spinal cord are the unmyelinated integration centers where information is processed.

Reflexes

Reflexes are fast, involuntary responses to stimuli. The most well known reflex is the patellar reflex, which is checked when a physicians taps on a patient’s knee during a physical examination. Reflexes are integrated in the gray matter of the spinal cord or in the brain stem. Reflexes allow the body to respond to stimuli very quickly by sending responses to effectors before the nerve signals reach the conscious parts of the brain. This explains why people will often pull their hands away from a hot object before they realize they are in pain.

Functions of the Cranial Nerves

Each of the 12 cranial nerves has a specific function within the nervous system.

• The olfactory nerve (I) carries scent information to the brain from the olfactory epithelium in the roof of the nasal cavity.
• The optic nerve (II) carries visual information from the eyes to the brain.
• Oculomotor, trochlear, and abducens nerves (III, IV, and VI) all work together to allow the brain to control the movement and focus of the eyes. The trigeminal nerve (V) carries sensations from the face and innervates the muscles of mastication.
• The facial nerve (VII) innervates the muscles of the face to make facial expressions and carries taste information from the anterior 2/3 of the tongue.
• The vestibulocochlear nerve (VIII) conducts auditory and balance information from the ears to the brain.
• The glossopharyngeal nerve (IX) carries taste information from the posterior 1/3 of the tongue and assists in swallowing.
• The vagus nerve (X), sometimes called the wandering nerve due to the fact that it innervates many different areas, “wanders” through the head, neck, and torso. It carries information about the condition of the vital organs to the brain, delivers motor signals to control speech and delivers parasympathetic signals to many organs.
• The accessory nerve (XI) controls the movements of the shoulders and neck.
• The hypoglossal nerve (XII) moves the tongue for speech and swallowing.

Sensory Physiology

All sensory receptors can be classified by their structure and by the type of stimulus that they detect. Structurally, there are 3 classes of sensory receptors: free nerve endings, encapsulated nerve endings, and specialized cells. Free nerve endings are simply free dendrites at the end of a neuron that extend into a tissue. Pain, heat, and cold are all sensed through free nerve endings. An encapsulated nerve ending is a free nerve ending wrapped in a round capsule of connective tissue. When the capsule is deformed by touch or pressure, the neuron is stimulated to send signals to the CNS. Specialized cells detect stimuli from the 5 special senses: vision, hearing, balance, smell, and taste. Each of the special senses has its own unique sensory cells—such as rods and cones in the retina to detect light for the sense of vision.

Functionally, there are 6 major classes of receptors: mechanoreceptors, nociceptors, photoreceptors, chemoreceptors, osmoreceptors, and thermoreceptors.  

• Mechanoreceptors. Mechanoreceptors are sensitive to mechanical stimuli like touch, pressure, vibration, and blood pressure.
• Nociceptors. Nociceptors respond to stimuli such as extreme heat, cold, or tissue damage by sending pain signals to the CNS.
• Photoreceptors. Photoreceptors in the retina detect light to provide the sense of vision.
• Chemoreceptors. Chemoreceptors detect chemicals in the bloodstream and provide the senses of taste and smell.
• Osmoreceptors. Osmoreceptors monitor the osmolarity of the blood to determine the body’s hydration levels.
• Thermoreceptors. Thermoreceptors detect temperatures inside the body and in its surroundings.

The endocrine system includes all of the glands of the body and the hormones produced by those glands. The glands are controlled directly by stimulation from the nervous system as well as by chemical receptors in the blood and hormones produced by other glands. By regulating the functions of organs in the body, these glands help to maintain the body’s homeostasis. Cellular metabolism, reproduction, sexual development, sugar and mineral homeostasis, heart rate, and digestion are among the many processes regulated by the actions of hormones.

Anatomy of the Endocrine System

Hypothalamus

The hypothalamus is a part of the brain located superior and anterior to the brain stem and inferior to the thalamus. It serves many different functions in the nervous system, and is also responsible for the direct control of the endocrine system through the pituitary gland. The hypothalamus contains special cells called neurosecretory cells—neurons that secrete hormones:

• Thyrotropin-releasing hormone (TRH)
• Growth hormone-releasing hormone (GHRH)
• Growth hormone-inhibiting hormone (GHIH)
• Gonadotropin-releasing hormone (GnRH)
• Corticotropin-releasing hormone (CRH)
• Oxytocin
• Antidiuretic hormone (ADH)

All of the releasing and inhibiting hormones affect the function of the anterior pituitary gland. TRH stimulates the anterior pituitary gland to release thyroid-stimulating hormone. GHRH and GHIH work to regulate the release of growth hormone—GHRH stimulates growth hormone release, GHIH inhibits its release. GnRH stimulates the release of follicle stimulating hormone and luteinizing hormone while CRH stimulates the release of adrenocorticotropic hormone. The last two hormones—oxytocin and antidiuretic hormone—are produced by the hypothalamus and transported to the posterior pituitary, where they are stored and later released.

Pituitary Gland

The pituitary gland, also known as the hypophysis, is a small pea-sized lump of tissue connected to the inferior portion of the hypothalamus of the brain. Many blood vessels surround the pituitary gland to carry the hormones it releases throughout the body. Situated in a small depression in the sphenoid bone called the sella turcica, the pituitary gland is actually made of 2 completely separate structures: the posterior and anterior pituitary glands.

Posterior Pituitary

The posterior pituitary gland is actually not glandular tissue at all, but nervous tissue instead. The posterior pituitary is a small extension of the hypothalamus through which the axons of some of the neurosecretory cells of the hypothalamus extend. These neurosecretory cells create 2 hormones in the hypothalamus that are stored and released by the posterior pituitary:

• Oxytocin triggers uterine contractions during childbirth and the release of milk during breastfeeding.
• Antidiuretic hormone (ADH) prevents water loss in the body by increasing the re-uptake of water in the kidneys and reducing blood flow to sweat glands.

Anterior Pituitary

The anterior pituitary gland is the true glandular part of the pituitary gland. The function of the anterior pituitary gland is controlled by the releasing and inhibiting hormones of the hypothalamus. The anterior pituitary produces 6 important hormones:

• Thyroid stimulating hormone (TSH), as its name suggests, is a tropic hormone responsible for the stimulation of the thyroid gland.
• Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex, the outer part of the adrenal gland, to produce its hormones.
• Follicle stimulating hormone (FSH) stimulates the follicle cells of the gonads to produce gametes—ova in females and sperm in males.
• Luteinizing hormone (LH) stimulates the gonads to produce the sex hormones—estrogens in females and testosterone in males.
• Human growth hormone (HGH) affects many target cells throughout the body by stimulating their growth, repair, and reproduction.
• Prolactin (PRL) has many effects on the body, chief of which is that it stimulates the mammary glands of the breast to produce milk.

Pineal Gland

The pineal gland is a small pinecone-shaped mass of glandular tissue found just posterior to the thalamus of the brain. The pineal gland produces the hormone melatonin that helps to regulate the human sleep-wake cycle known as the circadian rhythm. The activity of the pineal gland is inhibited by stimulation from the photoreceptors of the retina. This light sensitivity causes melatonin to be produced only in low light or darkness. Increased melatonin production causes humans to feel drowsy at nighttime when the pineal gland is active.

Thyroid Gland

The thyroid gland is a butterfly-shaped gland located at the base of the neck and wrapped around the lateral sides of the trachea. The thyroid gland produces 3 major hormones: 

• Calcitonin
• Triiodothyronine (T3)
• Thyroxine (T4)

Calcitonin is released when calcium ion levels in the blood rise above a certain set point. Calcitonin functions to reduce the concentration of calcium ions in the blood by aiding the absorption of calcium into the matrix of bones. The hormones T3 and T4 work together to regulate the body’s metabolic rate. Increased levels of T3 and T4 lead to increased cellular activity and energy usage in the body.

Parathyroid Glands

The parathyroid glands are 4 small masses of glandular tissue found on the posterior side of the thyroid gland. The parathyroid glands produce the hormone parathyroid hormone (PTH), which is involved in calcium ion homeostasis. PTH is released from the parathyroid glands when calcium ion levels in the blood drop below a set point. PTH stimulates the osteoclasts to break down the calcium containing bone matrix to release free calcium ions into the bloodstream. PTH also triggers the kidneys to return calcium ions filtered out of the blood back to the bloodstream so that it is conserved.

Adrenal Glands

The adrenal glands are a pair of roughly triangular glands found immediately superior to the kidneys. The adrenal glands are each made of 2 distinct layers, each with their own unique functions: the outer adrenal cortex and inner adrenal medulla.

Adrenal cortex

The adrenal cortex produces many cortical hormones in 3 classes: glucocorticoids, mineralocorticoids, and androgens.

• Glucocorticoids have many diverse functions, including the breakdown of proteins and lipids to produce glucose. Glucocorticoids also function to reduce inflammation and immune response.
• Mineralocorticoids, as their name suggests, are a group of hormones that help to regulate the concentration of mineral ions in the body.
• Androgens, such as testosterone, are produced at low levels in the adrenal cortex to regulate the growth and activity of cells that are receptive to male hormones. In adult males, the amount of androgens produced by the testes is many times greater than the amount produced by the adrenal cortex, leading to the appearance of male secondary sex characteristics.

Adrenal medulla

The adrenal medulla produces the hormones epinephrine and norepinephrine under stimulation by the sympathetic division of the autonomic nervous system. Both of these hormones help to increase the flow of blood to the brain and muscles to improve the “fight-or-flight” response to stress. These hormones also work to increase heart rate, breathing rate, and blood pressure while decreasing the flow of blood to and function of organs that are not involved in responding to emergencies.

Pancreas

The pancreas is a large gland located in the abdominal cavity just inferior and posterior to the stomach. The pancreas is considered to be a heterocrine gland as it contains both endocrine and exocrine tissue. The endocrine cells of the pancreas make up just about 1% of the total mass of the pancreas and are found in small groups throughout the pancreas called islets of Langerhans. Within these islets are 2 types of cells—alpha and beta cells. The alpha cells produce the hormone glucagon, which is responsible for raising blood glucose levels. Glucagon triggers muscle and liver cells to break down the polysaccharide glycogen to release glucose into the bloodstream. The beta cells produce the hormone insulin, which is responsible for lowering blood glucose levels after a meal. Insulin triggers the absorption of glucose from the blood into cells, where it is added to glycogen molecules for storage.

Gonads

The gonads—ovaries in females and testes in males—are responsible for producing the sex hormones of the body. These sex hormones determine the secondary sex characteristics of adult females and adult males.

• Testes: The testes are a pair of ellipsoid organs found in the scrotum of males that produce the androgen testosterone in males after the start of puberty. Testosterone has effects on many parts of the body, including the muscles, bones, sex organs, and hair follicles. This hormone causes growth and increases in strength of the bones and muscles, including the accelerated growth of long bones during adolescence. During puberty, testosterone controls the growth and development of the sex organs and body hair of males, including pubic, chest, and facial hair.
• Ovaries: The ovaries are a pair of almond-shaped glands located in the pelvic body cavity lateral and superior to the uterus in females. The ovaries produce the female sex hormones progesterone and estrogens. Progesterone is most active in females during ovulation and pregnancy where it maintains appropriate conditions in the human body to support a developing fetus. Estrogens are a group of related hormones that function as the primary female sex hormones. The release of estrogen during puberty triggers the development of female secondary sex characteristics such as uterine development, breast development, and the growth of pubic hair. Estrogen also triggers the increased growth of bones during adolescence that lead to adult height and proportions.

Thymus

The thymus is a soft, triangular-shaped organ found in the chest posterior to the sternum. The thymus produces hormones called thymosins that help to train and develop T-lymphocytes during fetal development and childhood. The T-lymphocytes produced in the thymus go on to protect the body from pathogens throughout a person’s entire life. The thymus becomes inactive during puberty and is slowly replaced by adipose tissue throughout a person’s life.

Other Hormone Producing Organs

In addition to the glands of the endocrine system, many other non-glandular organs and tissues in the body produce hormones as well.  

• Heart: The cardiac muscle tissue of the heart is capable of producing the hormone atrial natriuretic peptide (ANP) in response to high blood pressure levels. ANP works to reduce blood pressure by triggering vasodilation to provide more space for the blood to travel through. ANP also reduces blood volume and pressure by causing water and salt to be excreted out of the blood by the kidneys.
• Kidneys: The kidneys produce the hormone erythropoietin (EPO) in response to low levels of oxygen in the blood. EPO released by the kidneys travels to the red bone marrow where it stimulates an increased production of red blood cells. The number of red blood cells increases the oxygen carrying capacity of the blood, eventually ending the production of EPO.
• Digestive System: The hormones cholecystokinin (CCK), secretin, and gastrin are all produced by the organs of the gastrointestinal tract. CCK, secretin, and gastrin all help to regulate the secretion of pancreatic juice, bile, and gastric juice in response to the presence of food in the stomach. CCK is also instrumental in the sensation of satiety or “fullness” after eating a meal.
• Adipose: Adipose tissue produces the hormone leptin that is involved in the management of appetite and energy usage by the body. Leptin is produced at levels relative to the amount of adipose tissue in the body, allowing the brain to monitor the body’s energy storage condition. When the body contains a sufficient level of adipose for energy storage, the level of leptin in the blood tells the brain that the body is not starving and may work normally. If the level of adipose or leptin decreases below a certain threshold, the body enters starvation mode and attempts to conserve energy through increased hunger and food intake and decreased energy usage. Adipose tissue also produces very low levels of estrogens in both men and women. In obese people the large volume of adipose tissue may lead to abnormal estrogen levels.
• Placenta: In pregnant women, the placenta produces several hormones that help to maintain pregnancy. Progesterone is produced to relax the uterus, protect the fetus from the mother’s immune system, and prevent premature delivery of the fetus. Human chorionic gonadotropin (HCG) assists progesterone by signaling the ovaries to maintain the production of estrogen and progesterone throughout pregnancy.
• Local Hormones: Prostaglandins and leukotrienes are produced by every tissue in the body (except for blood tissue) in response to damaging stimuli. These two hormones mainly affect the cells that are local to the source of damage, leaving the rest of the body free to function normally.
• Prostaglandins cause swelling, inflammation, increased pain sensitivity, and increased local body temperature to help block damaged regions of the body from infection or further damage. They act as the body’s natural bandages to keep pathogens out and swell around damaged joints like a natural cast to limit movement.
• Leukotrienes help the body heal after prostaglandins have taken effect by reducing inflammation while helping white blood cells to move into the region to clean up pathogens and damaged tissues.

Physiology of the Endocrine System

Endocrine System vs. Nervous System Function

The endocrine system works alongside of the nervous system to form the control systems of the body. The nervous system provides a very fast and narrowly targeted system to turn on specific glands and muscles throughout the body. The endocrine system, on the other hand, is much slower acting, but has very widespread, long lasting, and powerful effects. Hormones are distributed by glands through the bloodstream to the entire body, affecting any cell with a receptor for a particular hormone. Most hormones affect cells in several organs or throughout the entire body, leading to many diverse and powerful responses.  

Hormone Properties

Once hormones have been produced by glands, they are distributed through the body via the bloodstream. As hormones travel through the body, they pass through cells or along the plasma membranes of cells until they encounter a receptor for that particular hormone. Hormones can only affect target cells that have the appropriate receptors. This property of hormones is known as specificity. Hormone specificity explains how each hormone can have specific effects in widespread parts of the body.

Many hormones produced by the endocrine system are classified as tropic hormones. A tropic hormone is a hormone that is able to trigger the release of another hormone in another gland. Tropic hormones provide a pathway of control for hormone production as well as a way for glands to be controlled in distant regions of the body. Many of the hormones produced by the pituitary gland, such as TSH, ACTH, and FSH are tropic hormones.

Hormonal Regulation

The levels of hormones in the body can be regulated by several factors. The nervous system can control hormone levels through the action of the hypothalamus and its releasing and inhibiting hormones. For example, TRH produced by the hypothalamus stimulates the anterior pituitary to produce TSH. Tropic hormones provide another level of control for the release of hormones. For example, TSH is a tropic hormone that stimulates the thyroid gland to produce T3 and T4. Nutrition can also control the levels of hormones in the body. For example, the thyroid hormones T3 and T4 require 3 or 4 iodine atoms, respectively, to be produced. In people lacking iodine in their diet, they will fail to produce sufficient levels of thyroid hormones to maintain a healthy metabolic rate. Finally, the number of receptors present in cells can be varied by cells in response to hormones. Cells that are exposed to high levels of hormones for extended periods of time can begin to reduce the number of receptors that they produce, leading to reduced hormonal control of the cell.

Classes of Hormones

Hormones are classified into 2 categories depending on their chemical make-up and solubility: water-soluble and lipid-soluble hormones. Each of these classes of hormones has specific mechanisms for their function that dictate how they affect their target cells.

• Water-soluble hormones: Water-soluble hormones include the peptide and amino-acid hormones such as insulin, epinephrine, HGH, and oxytocin. As their name indicates, these hormones are soluble in water. Water-soluble hormones are unable to pass through the phospholipid bilayer of the plasma membrane and are therefore dependent upon receptor molecules on the surface of cells. When a water-soluble hormone binds to a receptor molecule on the surface of a cell, it triggers a reaction inside of the cell. This reaction may change a factor inside of the cell such as the permeability of the membrane or the activation of another molecule. A common reaction is to cause molecules of cyclic adenosine monophosphate (cAMP) to be synthesized from adenosine triphosphate (ATP) present in the cell. cAMP acts as a second messenger within the cell where it binds to a second receptor to change the function of the cell’s physiology.
• Lipid-soluble hormones: Lipid-soluble hormones include the steroid hormones such as testosterone, estrogens, glucocorticoids, and mineralocorticoids. Because they are  soluble in lipids, these hormones are able to pass directly through the phospholipid bilayer of the plasma membrane and bind directly to receptors inside the cell nucleus. Lipid-soluble hormones are able to directly control the function of a cell from these receptors, often triggering the transcription of particular genes in the DNA to produce “messenger RNAs (mRNAs)” that are used to make proteins that affect the cell’s growth and function.