The skin is a tough, elastic structure covering the entire body (Figure 2-9). It is made up of two principal layers, the epidermis or cuticle and the dermis or true skin. The epidermis, which overlies the dermis, is itself composed of a superficial layer and an inner layer. The superficial or horny layer consists of dead cells that are constantly being worn off. These are replaced from the living cells that form the inner layer. The dermis is the thicker part of the skin, and consists of connective tissue containing blood vessels, nerve endings, sweat glands, sebaceous glands, and hair follicles. The dermis is held in place by a layer of areolar connective tissue.
a. Protection. The skin protects underlying structures by acting as a mechanical barrier. When the skin is broken, bacteria may invade the body through the opening.
b. Regulation of Body Temperature. The skin regulates the body temperature by controlling heat loss in two ways:
(1) The blood vessels in the skin change in size; they dilate and bring warm blood to the surface to increase heat loss, and they constrict to decrease heat loss.
(2) The skin produces sweat which, when it evaporates, cools the body surface.
c. Sensory Perception. The skin acts as an organ of perception. It contains sensory nerve endings which are specialized to detect heat, cold) pressure (touch), and pain.
d. Excretion. The excretion of waste products through the skin is a function of the sweat glands that open by a duct onto the skin surface. The opening is called a pore. These glands are distributed in large numbers over the body and secrete an average of a quart of perspiration each day; although, the amount varies considerably, depending on the temperature and humidity of the atmosphere, and the amount of exercise performed by the individual. Perspiration is continuous, but it may be so slow and the sweat may evaporate so quickly that it is imperceptible. Sweat consists chiefly of water (99 percent), with small quantities of salts and organic materials which are waste products. Skin also secretes a thick substance, sebum. This material is the product of the sebaceous glands, and its purpose is to lubricate the skin and keep it soft and pliable.
e. Absorption. Although not one of its normal functions, the skin is capable of absorbing water and other substances. Physicians take advantage of this fact by prescribing local application of certain drugs.
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