The Skin

The skin is the largest organ of the body, covering 18 square feet and weighing about seven pounds in an average adult. The skin acts as a waterproof barrier that affords protection from invasion by dirt, bacteria, and other harmful substances and helps to regulate body temperature.

The skin is composed of three layers -- the epidermis, the dermis, and the subcutaneous layer.


The outermost layer of skin is the epidermis. The epidermis contains pigment cells that determine skin color and shield the skin against damaging sun rays. Specialized cells within this layer manufacture keratin, a tough substance also found in hair and nails.

Epidermal cells are continuously being worn away and replaced. This reconstruction process is usually invisible. Since the outer skin layer repairs itself quickly, any injury to the epidermis rarely causes injury to the body as a whole.


The dermis contains blood vessels, nerve endings, hair follicles, sweat glands, and sebaceous (oil) glands. Damage occurring at this level can send infection into the bloodstream and throughout the body.

Within the dermis, blood vessels and sweat glands help the body regulate heat. If the temperature of blood rises, the brain stimulates secretion by the sweat glands. Sweat then flows to the surface of the skinthrough ducts and cools the skin by evaporating. The sebaceous glands prevent excessive evaporation by coating the surface of the skin with an oily substance called sebum.

Subcutaneous Layer

Beneath the dermis is the subcutaneous layer, in which the sweat glands originate and fat is stored. This layer also supports the blood vessels and nerves that supply the outer layers of the skin.

Because the dermis and the subcutaneous layer are rich with nerve endings, the skin is also a sensory organ. Nerves throughout these layers transmit tactile perceptions to the brain.