Skin
The skin of a vertebrate is far more than simply an elastic container of epithelial cells encasing the body's muscles, blood, and bones. Instead, it is a dynamic organ that has many functions:
Skin is a protective barrier. It keeps out microorganism that would otherwise infect the body. Because skin is water proof, it keeps the fluids of the body in and other fluids out; when you soak in a bathtub, your body absorbs little or no water. Skin cells also contain a pigment called melanin, which absorbs potentially damaging ultraviolet radiation from the sun.Skin provides a sensory surface. Sensory nerve ending in skin act as your body's pressure gauge, telling you how gently to garess a loved one, how hard to hold a pencil. Other sensors embedded in skin detect pain, heat, and cold. Skin is the body's point of contact with the outside world.Skin compensates for body movement. Skin stretches when you reach for something and contracts quickly when you stop reaching. It expands when you grow and shrinks when you lose weight.
Skin helps control the body's internal temperature. When themperature is cold, the blood vessels in the skin contract so that less of the body's heat is lost to the surrounding air. When it is hot, these same vessels expand, and glands in the skin release sweat, whose evaporation cools the body surface.Skin is the largest organ of the vertebrate body. In an adult human, 15% of the total body weight is skin. Much of the multifunctional role of skin reflects the fact that crammed in among its cells are many other specialized cells. One square centimeter of human skin contains 200 nerve endings, 10 hair and muscles, 100 sweat glands, 15 oil glands, 3 blood vessels, 12 heat receptors, 2 cold receptors, and 25 pressure-sensing receptors.