Physiology is the branch of biology that deals with the parts of the body, their functions, and the various bodily processes. Human physiology deals with the human body.

Cells, Tissues, Organs, and Organ Systems

In the human body, there are many different kinds of cells. Each kind is specialized for the primary function it performs. Examples are fat cells, skin cells, muscle cells, bone cells, and nerve cells. Groups of cells arrange themselves into tissues, and various tissues work together to form organs, such as the skin, liver, heart, gallbladder, and intestines. Organs work together to form organ systems. Organ systems include the muscular system, skeletal system, skin or integumentary system, respiratory system, digestive system, circulatory system, lymphatic system, immune system, excretory system, nervous system, endocrine system, and reproductive system.

Muscular System

The muscular system allows movement and locomotion. The muscular system helps you make body movements and supports the body in its activities. Muscles are involved in breathing, your heart beating, and the working of your digestive system.

Some actions are controlled by voluntary muscles, such as making a face, showing your biceps, or walking. Other muscle systems are involuntary, such as those that control breathing, your heart beating, and your digestive process.

Skeletal muscles help move the bones. They are attached to the bone by bands of tissue called tendons. Skeletal muscles work in pairs; when one muscle of the pair contracts, the other muscle relaxes. Cardiac muscle is found in the heart. Smooth muscles are found in some of your internal organs, such as your intestines and bladder.

Skeletal System

The skeletal system is a living system that provides shape and support to your body. It is built to protect your inner organs and to provide attachment points for muscles. The skeletal system provides a rigid framework for movement. It supports and protects the body and body parts, produces blood cells, and stores minerals. In lower animals, such as a grasshopper, the skeleton might be on the outside. This is called an exoskeleton.

Vertebrates have developed an internal mineralized endoskeleton. The endoskeleton is made up of bone and cartilage. Muscles are on the outside of the endoskeleton. Although our endoskeleton is mostly bone, some parts of our body are made of cartilage, such as the trachea, nose, and ears. The skeleton and muscles function together as the musculoskeletal system. Calcium and phosphorus are important components of bone; these elements make bone hard. Osteoblasts are bone-forming cells.

Places where your bones come together are called joints. Joints are held together by bands of tissue called ligaments. Bones move at the joints. There are three major types of joints: ball and socket joints, like the shoulder and hip; pivot joints, like the elbow; and hinge joints, like the knee. These joints, working with the muscles and tendons, allow the body to move in certain ways.

Skeletal System

Integumentary System

Skin or integument is your outermost protective layer. The skin is the largest organ in your body. It protects you from losing water and from the invasion of foreign organisms and viruses. Special nerve cells in the skin help send information to the brain, helping you to feel hot and cold, texture, softness, and pain. Your skin has hair, oil glands, and sweat glands.

Skin helps to regulate your body temperature by expanding or constricting blood vessels and through the operation of the sweat glands. The sweat glands move perspiration or sweat onto the skin, where evaporation takes place and cools the skin.

The skin has three layers. The epidermis is the outer, thinner layer of skin. Underneath the epidermis, skin cells are continually being produced. The next layer is the dermis, containing blood vessels, nerves, muscles, and oil and sweat glands. Underneath the dermis is a layer of fat. This is where a lot of fat is stored as you gain weight.

Skin gets its color from melanin. People with different color skin have different amounts of melanin. The darker the skin, the more melanin the skin has. Skin also helps produce vitamin D when it is exposed to ultraviolet light.

Skin bruises when tiny blood vessels beneath the skin burst.

Respiratory System

The respiratory system takes in oxygen and moves out waste material of carbon dioxide. Our lungs perform this job by breathing in and out (gills do this in fish). This system includes the lungs, pathways connecting them to the outside environment, and structures in the chest involved with moving air in and out of the lungs.

Respiratory System

When we breathe in, air enters the body through the nose, where it is warmed, filtered, and passed through the nasal cavity. Air passes the pharynx (which has the epiglottis that prevents food from entering the trachea). The upper part of the trachea contains the larynx. After passing the larynx, the air moves into the bronchi, which carry air in and out of the lungs.

The bronchi are lined with epithelium and mucus-producing cells. The bronchi branch into smaller and smaller tubes known as bronchioles. The bronchioles terminate in little sacks known as alveoli. Alveoli are surrounded by a network of thin-walled capillaries. The exchange of oxygen takes place there, between the alveoli and the capillaries. The capillaries take in the oxygen. The hemoglobin in the blood picks up the oxygen and carries it off to various parts of the body. At the same time, waste material is transferred to the alveoli and eventually out of our bodies during exhalation.

It is important to note that breathing is different from respiration. Breathing is the physical action of moving the diaphragm up and down, which allows air to enter our lungs. Respiration is how our bodies use the oxygen from the air we inhale and eliminate the carbon dioxide when we exhale.

Digestive System

Digestion is accomplished by mechanical and chemical means, breaking food into particles small enough to pass into the bloodstream. The digestive process begins as soon as you put food into your mouth. Mechanical breakdown begins in the mouth by chewing (teeth) and moving the food around with your tongue. The food interacts with your saliva to begin a chemical breakdown. This mixture of food and saliva is then pushed into the pharynx and esophagus when you swallow. The esophagus is a long tube with muscles that contract and move the food to the stomach. The stomach is a sack or bag. It breaks down the food by both mechanical and chemical means. The stomach mixes the food by a process called peristalsis. Peristalsis is merely waves of muscle contractions. The stomach releases enzymes and hydrochloric acid to break down the food even further. Eventually the food moves to your small intestine.

The small intestine is narrow in diameter but very long in length. The upper part of the small intestine is the duodenum. This is where most digestion takes place. Bile is introduced from your liver. Bile breaks up the fat particles. Next comes the pancreas, which introduces pancreatic fluid. This fluid promotes the chemical digestion of carbohydrates, proteins, and fats. The fluid also neutralizes the stomach acid and makes insulin. Absorption of the food takes place in the small intestine.

Food that is not digested continues to move through your system into the large intestine by means of peristalsis. The large intestine absorbs water, making the contents more solid. Eventually the rectum and anus control the release of the semisolid waste called feces.

Digestive System

Circulatory System

The circulatory system is one of the transportation systems in your body. The main job of the circulatory system is to move oxygen, carbon dioxide, nutrients, waste products, immune components, and hormones through your body. The circulatory system is composed of vessels and muscles that control the flow of blood around the body. This process of blood flowing around the body is called circulation. The main components of the circulatory system are the heart, arteries, capillaries, and veins.

There are three types of circulation, coronary circulation, pulmonary circulation, and systemic circulation. Coronary circulation is the circulation of blood within the heart itself by the coronary veins and arteries. If this circulation is blocked, it could result in a heart attack. Pulmonary circulation is the flow of blood from the heart to the lungs and back. Systemic circulation is the blood (with oxygen) moving through your body to your important organs.

The heart is a strong muscle that operates as the engine of the circulatory system. It has four compartments that are called chambers. The two upper chambers are called atriums and the two lower chambers are called ventricles. When your heart beats, the two ventricles contract at the same time, followed by the two atria. Blood flows from an atrium to a ventricle and then from the ventricle to a blood vessel. The pulmonary valves keep the blood from flowing backward. There is a wall between the two atria and the two ventricles, keeping blood that has a lot of oxygen apart from blood that does not have as much oxygen.

Blood returns from the body to the right atrium of the heart. At this point, the blood is low on oxygen. When the right atrium contracts, it moves the blood to the right ventricle. The blood then moves via the pulmonary artery to the lungs, where it picks up oxygen and eliminates waste (carbon dioxide). The oxygenated blood leaves the lungs and comes into the heart through the left atrium. Blood leaves the heart from the left ventricle and goes into the biggest artery, called the aorta. Fresh blood from the aorta goes to various parts of your body, including the brain, which needs a constant supply of oxygen. Blood returns to the heart through the veins, starting the cycle over. Arteries are blood vessels that carry blood away from the heart. Veins carry blood to the heart. Arteries and veins are connected by thousands of miles of tiny vessels called capillaries.

Circulatory System

Blood   About five liters of blood flows through your body, delivering essential elements and removing harmful wastes. Blood transports oxygen from the lungs to body tissues and carbon dioxide from body tissues to the lungs. It also transports disease-fighting substances to the tissue and waste to the kidneys.

Blood contains red blood cells and white blood cells. These cells are responsible for nourishing and cleansing the body. Red blood cells are shaped like disks and contain hemoglobin, which carries oxygen and carbon dioxide. Unlike other cells, they do not have a nucleus. White blood cells fight bacteria, viruses, and other intruders in your body. The white blood cells are carried to the place where an intruder has invaded and go into that tissue. Their job is to destroy the bacteria or viruses.

Cell fragments called platelets are carried along with the red and white blood cells. Platelets plug holes in small blood vessels to stop bleeding. They help to clot blood.

Red and white blood cells and platelets are carried by a fluid called plasma. About 55 percent of the blood's volume comes from plasma. Nutrients, minerals, and oxygen are carried by the plasma to various parts of the body. Waste products are also carried by the plasma.

There are four different blood types: A, B, AB, and O. Types A, B, and AB have certain chemical tags called antigens. Type O has no antigens. Blood also has antibodies that destroy or neutralize substances that do not belong there. This prevents certain blood types from being mixed. The following chart indicates who can receive and give certain blood types.

People with type O blood are called universal donors; those with type AB are called universal receivers.

Your blood also has Rh factors, making it even more unique. The Rh factor is an additional chemical tag. If the blood has the Rh factor, it is labeled Rh-positive. If the factor is not present, the blood is called Rh-negative. If the wrong blood is given to a person, the blood will try to destroy the other person's blood. That's why it is important to know your blood type.

Lymphatic System

The lymphatic system is composed of lymph vessels, lymph nodes, and certain organs. The system absorbs excess fluids from the body and returns them to the bloodstream. It also absorbs fat and transports it to the heart. The fluid contains lymphocytes, which are a type of white blood cell that tries to destroy disease-causing organisms.

The lymphatic system is a lot like the circulatory system. The fluid lymph passes through lymph nodes that remove any microorganisms and foreign materials. Lymph nodes generally occur in clusters in the neck, armpits, and groin. If you get sick with an infection, the lymphocytes fill the lymph nodes, and your lymph nodes may feel tender and swollen.

There are three organs that are part of the lymphatic system. These are the tonsils, the thymus, and the spleen. Your tonsils help keep out invaders that try to come in through your nose and mouth. The thymus makes lymphocytes. The spleen filters the blood and removes worn-out or damaged red blood cells. Cells in the spleen destroy bacteria and other invaders.

Immune System

The immune system defends our bodies from invading microorganisms and viruses called pathogens, as well as from cancerous cell growth. Immune-system components are grouped into first-line defenses and second-line defenses. First-line defenses include your skin and your respiratory, digestive, and circulatory systems.

Pathogens can't get through your skin unless it is cut or broken, but they can get though your mouth, nose, and eyes. The respiratory system uses cilia, little hairlike structures, and mucus to trap pathogens. When you cough or sneeze, you are expelling some mucus that contains pathogens. The digestive system uses saliva, enzymes, hydrochloric acid, and other substances to get rid of bacteria that can be harmful to you. The circulatory system uses white blood cells to surround and destroy foreign organisms and chemicals. Temperature destroys some organisms, so if your white blood cells cannot do the job fast enough, you might get a fever.

Second-line defenses are specific to the disease. Molecules that that are foreign to your body are called antigens. When your body determines that a foreign molecule has invaded, special lymphocytes, called T cells, attack. Special T cells stimulate other lymphocytes called B cells to form antibodies. Antibodies are made in response to a specific antigen.

With certain diseases, a lot of extra antibodies are formed, so that when your disease is cured, a few antibodies hang around and stay on watch. If the pathogen enters your body again, these antibodies can reproduce very rapidly and eliminate the disease. That's why some diseases, like chicken pox, you get only once. This is an example of active immunity.

Passive immunity occurs when you are vaccinated against a disease. A vaccination injects a type of antigen that gives you active immunity against the disease. It does this by stimulating the production of antibodies. Vaccinations are specific to one kind of virus or bacteria. For example, there is a new flu vaccine every year because the virus is different each year. Common vaccines include those for measles, diphtheria, tetanus, mumps, rubella, and whooping cough.

Diseases caused by bacteria include tetanus, tuberculosis, strep throat, and bacterial pneumonia. Viruses cause colds, influenza (flu), measles, polio, mumps, and smallpox. Antibiotics can cure some bacterial diseases, but not viral diseases.

Excretory System

The excretory system removes waste. It removes undigested material through the digestive system by way of the large intestine. It removes waste gases through the circulatory and respiratory systems. It removes salts through the skin when we sweat. It removes excess water and waste through the urinary system. The urinary system is responsible for maintaining the fluid levels in our bodies.

The kidneys play a major role in the excretory system. They are two bean-shaped organs that are responsible for filtering blood that contains waste from the cells. Once the blood has been purified by tiny filtering units called nephrons, it is returned to the circulatory system. The leftover water from this process is called urine. It is collected in the bladder and then eliminated through the urethra during urination. Persons who have kidney disease need to have dialysis to remove the waste from their blood. Without waste removal, a person will die.

Nervous System

The nervous system coordinates and controls such actions as memory, learning, and conscious thought. The nervous system also maintains such functions as heartbeat, breathing, and control of involuntary muscle actions. It is the most complex and delicate of all our body systems.

The largest organ in the nervous system is the brain. The brain is a sort of control center, as it sends and receives messages through a network of nerves. It is made up of a hundred billion neurons, or brain cells. The brain has three major parts: the cerebrum, the cerebellum, and the brain stem.

The cerebrum is the largest part of the brain; it takes care of our thinking processes. The outer layer of the cerebrum is called the cortex and has a lot of ridges and grooves. More ridges and grooves allow more complex thinking to occur.

The cerebellum is the second-largest part of the brain. Its job is to coordinate our muscle movements and maintain normal muscle tone and posture. The cerebellum coordinates our balance while walking, riding a bike, and so on.

The brain stem is closest to the spinal cord. It has three parts: the midbrain, the pons, and the medulla. The midbrain and pons coordinate various parts of the brain so that it acts together. The medulla is involved in coordinating our heartbeat, breathing, blood pressure, and the reflex centers for vomiting, coughing, sneezing, swallowing, and hiccupping.

Nervous System

The hypothalamus regulates thirst, hunger, body temperature, water balance, and blood pressure, and links the nervous system to the endocrine system.

Nervous System

The spinal cord is a thick bundle of nerves running down the center of the spine. Along the spinal cord, smaller bunches of nerves branch out. From these bunches, still smaller bundles of nerves branch out again. Eventually they reach every part of the body. The spinal cord is protected by a column of vertebrae—part of the skeletal system.

The brain and the spinal cord make up the central nervous system. The nerves outside the central nervous system are called the peripheral nervous system. These are the nerves in your head and the nerves that come out from the spinal cord. The peripheral nervous system connects the central nervous system to the rest of the body. The peripheral nervous system has two parts: the somatic system and the autonomic system. The somatic system controls voluntary movements, like walking, running, and swiveling your hip. The autonomic system controls involuntary movements, such as heartbeat, breathing, digestion, and so on.

Messages that are transported through the nervous system are conducted by the nerve cells. Each microscopic nerve cell, or neuron, has a blob-shaped main part, the cell body, and thin, spiderlike dendrites. It has one lone fiber called the axon. The axon's branched ends have little bulbs that almost touch adjacent nerve cells. The spaces between the nerve cells are called synapses. Nerve signals travel in one direction only along the axon and jump across synapses to other nerve cells.

Nervous System

Endocrine System

Endocrine glands secrete hormones that regulate body metabolism, growth, and reproduction. These organs are not in contact with each other, but they communicate through chemical messages transported by the circulatory system. The preceding table lists the major glands in the endocrine system, the hormone(s) each one produces, and what the hormones do. On the ASVAB test, you probably will not need to know the specific hormones that are produced, but just in general what each gland does.

Reproductive System

The purpose of the reproductive system is to continue the species for another generation. The organs of this system, called gonads, produce gametes that combine in the female system to produce the next generation. The male gonads are the testes, which produce sperm and male sex hormones. The female gonads are the ovaries, which produce eggs (ova) and female sex hormones. The sperm fertilizes the egg, and reproduction begins. Fertilization takes place when the sperm, using its flagellum (a sort of whip-like tail), moves through fluid to reach the egg. Each egg or sperm contains 23 chromosomes. These chromosomes carry DNA, which contributes to forming a new individual and determines the person's traits or characteristics.

Sex cells divide by a process called meiosis. Before this process begins, the nucleus divides twice, creating four cells, each with half the number of chromosomes of the original cell. So these cells have 23 chromosomes, half of what is needed. When a sperm and an egg combine, the fertilized egg contains the full complement of 46 chromosomes. These new cells keep dividing, growing, and developing over time, eventually creating a baby.