Human Physiology Study Guide for McGraw-Hill's ASVAB (page 3)

By — McGraw-Hill Professional
Updated on Jun 26, 2011

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.

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