Skin

2 Pages 585 Words

Skin, in anatomy, protective organ covering the external surface of the body and merging, without break, at the various body openings, with the mucous membranes of the alimentary and other canals. The skin forms a protective barrier against the action of physical, chemical, and bacterial agents on the deeper tissues and contains the special end organs for the various sensations commonly grouped as the sense of touch. Through the activity of its sweat glands and blood vessels, it is important in maintaining body temperature. One square inch (6.5 sq cm) of skin contains up to 4.5 m (15 ft) of blood vessels, which have as one of their functions the regulation of body temperature.
When the body temperature rises, radiant energy, or heat, is lost due to vascular dilation and increased blood flow to the skin surface. When the temperature is low, blood vessels constrict to reduce blood flow and subsequent heat loss through the skin. Each square inch of skin also contains hundreds of sweat glands that are controlled by a heat regulation center in the brain. These glands secrete moisture, which evaporates, cools the body surface, and helps maintain normal body temperature. In this capacity the skin acts as an excretory organ. The skin is elastic, and except for a few areas such as the palms, soles, and ears, is loosely attached to the underlying tissues. The color of the skin varies normally with the amount of pigment deposited in the skin cells as governed by heredity and by exposure to sunlight. The color also varies in disease because of difference in pigment, as in Addison's disease, or because of pigmented substances carried to it by the blood as in jaundice. In certain places the outer layers of the skin are modified to produce the hair and nails. The skin varies in thickness from 0.5 mm (0.02 in) on the eyelids to 4 mm (0.17 in) or more on the palms and soles.
In structure the skin is composed of two distinct layers. The outer layer, ca...