Skin is the organ that covers the bodies of human beings and many other animals. In human beings, the skin protects the body in a wide variety of ways. For example, the skin is almost completely waterproof and so prevents the escape of the fluids that bathe body tissues. It also prevents bacteria and chemicals from entering most parts of the body and is an important part of the body's defense against disease. The skin protects underlying tissues from harmful rays of the sun.
In addition, the skin helps keep the internal temperature of the body within normal levels. Glands in the skin release sweat when a person becomes overheated. The sweat evaporates and so cools the body. When a person becomes too cool, the body retains heat by narrowing the blood vessels in the skin. As a result, the flow of blood near the surface of the body decreases, and the body gives off less heat. The skin has many nerve endings that are sensitive to cold and heat, as well as pain, pressure, and touch.
The skin is the largest organ of the human body. If the skin of a 150-pound (68-kilogram) adult male were spread out flat, it would cover about 20 square feet (1.8 square meters).
