The circulatory and respiratory systems. All parts of the body must have nourishment and oxygen in order to function and grow, and their waste products must be removed before they accumulate and poison the body. The circulatory system distributes needed materials and removes unneeded ones. It is made up of the heart, blood vessels, and blood. The blood is also part of the body's defense system. It has antibodies and white blood cells that protect the body against foreign invaders.
The heart is a muscle that is divided into two nearly identical halves: one half receives blood from the lungs and sends it to the rest of the body, the other half sends blood that has traveled through the body back to the lungs. When the heart muscle contracts, the blood is forced out into arteries and enters small capillaries. Blood returns to the heart through veins.
Materials enter and leave the blood across the thin walls of capillaries, which are located near every cell of the body. In almost every case, blood leaving a group of capillaries travels to the heart and then to the lungs for more oxygen before it returns to the capillaries. The one exception is blood that has traveled through capillaries in the digestive system. The vein from this system, called the portal vein, carries blood directly to the liver, where nutrients are stored before the blood returns to the heart.
Some of the fluid that surrounds cells does not reenter the blood vessels directly. This fluid, called lymph, returns to the heart by way of another system of channels the lymph vessels. Lymph nodes along these vessels filter the fluid before it reenters the blood. The spleen is a large lymphatic organ, located under the stomach, that filters the blood.
The respiratory system takes in oxygen from the air and expels carbon dioxide and water vapor. Air enters the nose and mouth and travels through the larynx, or voice box, and trachea, or windpipe. The trachea divides to enter each of the two lungs and then divides more than 20 times to form a very large number of small air spaces. Oxygen from the air enters the blood through capillaries in the walls of these air spaces, and the blood releases carbon dioxide into the air spaces to be exhaled.

The digestive system consists of a tube extending from the mouth to the anus. In it, food and fluids are taken in, moved through the body, and broken down into small molecules that are absorbed into the circulatory system. This breakdown, known as digestion, is both a mechanical and a chemical process.
Food enters through the mouth, where chewing and saliva start to break it up and make it easier to swallow. Next, the food travels down through the esophagus to the stomach. Contractions of the stomach's muscular wall continue to break down the food mechanically, and chemical digestion continues when acid and enzymes are secreted into the stomach cavity.
The liquified food gradually passes into the small intestine. In the first part of the small intestine, called the duodenum, enzymes from the pancreas are added. These enzymes complete the chemical breakdown of the food. The digestion of fat is aided by bile, which is made in the liver and stored in the gall bladder. The small intestine of an adult is about 21 feet (6.4 meters) long. Most of its length is devoted to absorbing the nutrients released during these digestive activities.
The liquid remainder of the food enters the large intestine, or colon, which is about 12 feet (3.7 meters) long. It is more than twice as wide as the small intestine. In the large intestine most of the fluid is absorbed, and the relatively dry residues are expelled.

The urinary system maintains normal levels of water and of certain small molecules such as sodium and potassium in the body. It does this by passing blood through the kidneys, two efficient filtering organs that get rid of any excess of various molecules and conserve those molecules that are in short supply.
The fluid that leaves the kidneys, known as urine, travels through a tube called the ureter to the bladder. The bladder holds the urine until it is voided from the body through another tube, the urethra.

The glandular system. The two systems that control body activities are the glandular system and the nervous system. The glandular system exerts its control by means of chemical messengers called hormones. Hormones are produced by a variety of endocrine glands, which release the hormones directly into the blood stream.
A major gland is the pituitary, which is located under the brain in the middle of the head. It produces at least eight hormones, which affect growth, kidney function, and development of the gonads, or sex organs. Because some of the pituitary's hormones stimulate other glands to produce their own hormones, the pituitary is called the master gland.
Another gland, the thyroid, is located between the collar bones. Its hormone controls the rate of the body's metabolism. The sex organs (ovaries and testes) make the sex cells and also make hormones that control certain characteristics of males and females. Located on top of each kidney is the adrenal gland, which produces cortisone and epinephrine (also called adrenaline). The pancreas produces not only digestive enzymes but also insulin and glucagon, which control the body's use of sugar and starches.

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Excerpted from Compton's Interactive Encyclopedia
Copyright (c) 1994, 1995 Compton's NewMedia, Inc. All Rights Reserved