Anatomy & Physiology

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About 75 to 80 percent of an adult's body consists of slightly salty water; the rest is made up of chemical compounds, many of them unique to human beings. These various compounds are arranged to form hundreds of different kinds of cells, the body's smallest, most basic units.

All human life begins with the fusing of two cells, and the subsequent division and multiplication of cells to form a complete body follows the same general blueprint even though no two people are exactly alike. The average body contains 80 to 100 trillion cells, each programmed to grow, carry out a specific function, and even replicate itself. But, with the exception of blood cells, none function independently; instead, similar cells join together to form specific types of tissue — muscle, nerve, bone, and so forth. Each body organ is made up of a collection of related tissues. Finally, organs are organized into different body systems.

The respiratory and circulatory systems work in concert to provide a constant supply of oxygen to every cell in the body.

The renal system filters wastes from the blood, which are then excreted through the urinary tract. The kidneys produce hormones that are instrumental in controlling blood pressure and the manufacturing of red blood cells.

The reproductive organs do more than ensure the survival of the species by producing future generations; they also make the hormones that give males and females their respective physical characteristics.

The body's 600 or so voluntary muscles work with the skeletal and nervous systems to make movement possible. Involuntary muscles are instrumental in the smooth functioning of all the other body systems.

The brain, spinal cord, and peripheral nerves that make up the nervous system function as the body's communications network. The nervous system controls all other organ systems; it also is linked directly with the eyes, ears, and other sensory organs.

The endocrine system is made up of various glands that secrete hormones, the chemical messengers that control every bodily process. Hormones are also produced by such other organs as the stomach, lungs, kidneys, and heart.

The organs of the digestive system form a hollow tube that extends from the mouth to the anus. As good passes through this tube, the various organs break it down into molecules that the body can turn into energy and new tissue.

The adult skeleton contains more than 200 bones, giving the body its form and ability to move. Bones also store calcium and other essential minerals and serve as manufacturing plants for blood cells.

The skin acts as a protective barrier against a hostile outside environment. It also manufactures vitamin D, helps regulate body temperature, and is essential to the sense of touch.