Anatomists find it useful to divide the human body into eight systems: the skeleton, the muscles, the circulatory and respiratory systems, the digestive system, the urinary system, the glandular system, the nervous system, and the skin. The principal parts of each of these systems are described and illustrated in this article. (For information about the reproductive system, see Reproductive System.)

The skeleton is made of bones, joints between bones, and cartilage. Its function is to provide support and protection for the soft tissues and the organs of the body and to provide points of attachment for the muscles that move the body. There are 206 bones in the human skeleton. They have various shapes long, short, cube-shaped, flat, and irregular. Many of the long bones have an interior space that is filled with bone marrow, where blood cells are made.

   A joint is where bones are joined together. The connection can be so close that no movement is possible, as is the case in the skull. Other kinds of joints permit movement: either back and forth in one plane as with the hinge joint of the elbow or movement around a single axis as with the pivot joint that permits the head to rotate. A wide range of movement is possible when the ball-shaped end of one bone fits into a socket at the end of another bone, as they do in the shoulder and hip joints. In order to make movement easier, the ends of the bones are covered with a layer of cartilage and there is fluid in the space between them. Joints are held together by bands of connective tissue called ligaments.

   Cartilage is a more flexible material than bone. It serves as a protective, cushioning layer where bones come together. It also connects the ribs to the breastbone and provides a structural base for the nose and the external ear. An infant's skeleton is made of cartilage that is gradually replaced by bone as the infant grows into an adult.

The muscles allow the body to move, and their contractions produce heat, which helps maintain a constant body temperature. Striated muscles can be consciously controlled. The ends of these muscles are attached to different bones by connective tissue bands, called tendons, so that when the muscle contracts, one bone moves in relation to the other. This makes it possible to move the whole body, as when walking, or to move just one part of the body, as when bending a finger.

   Contractions of the heart and smooth muscles are not under conscious control. Smooth muscles are found in the walls of organs such as the stomach and the intestines and serve to move the contents of these organs through the body.

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.

The nervous system  the brain, the spinal cord, and the nerves also controls body activities. The lower parts of the brain control basic functions such as breathing and heart rate as well as body temperature, hunger, and thirst. Above these regions are the centers for sight, sound, touch, smell, and taste, and the regions that direct voluntary muscular activities of the arms and legs. Performed here are the higher functions of integrating and processing information.

   The brain receives and sends information by means of nerves, many of which lie partly in the spinal cord. The spinal cord is protected by the spinal column. Nerves enter and leave the spinal cord at each level of the body, traveling to and from the arms, legs, and trunk. These nerves bring information from the various sense organs. The information is processed by the brain, and then messages are carried back to muscles and glands throughout the body.

The skin is a complete layer that protects the inner structures of the body, and it is the largest of the body's organs. It keeps out foreign substances and prevents excessive water evaporation. The nerves in the skin provide tactile information. The skin also helps keep the body's temperature close to 98.6  F (about 37  C): heat is conserved by reducing blood flow through the skin or is expended by increasing blood flow and by evaporation of sweat from the skin. Hair and nails are accessory structures of the skin.