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How Skin Works

Public Domain

Public Domain

Skin is the largest organ of the human body. In an adult it covers a surface area averaging about 2,750 square inches, makes up about 7 percent of the total body weight, and receives roughly one third of the fresh blood pumped from the heart.

Versatile in its range of functions, human skin is a waterproof fabric that serves as a first line of defense against injury or invasions by hostile organisms and also has important roles to play as a sensory organ, as an agent of secretion and excretion, and as a modifier of body temperature.

Microscopic examination of the epidermis, or outer skin, reveals five layers through which cells must migrate to replace the dead cells shed from the surface of the skin. In the deepest level of the epidermis, the stratum basale, cells constantly divide to provide the steady supply of fresh cells forming the next layer, the stratum germinativum.

As the cells move farther away from the source of blood and nutrients in the underlying dermis they gradually degenerate-by filling with granules of protein waste-and die. Next they form the stratum granulosum, and then the shiny, almost transparent stratum lucidum.

Flattened and laden with keratin- the durable protein of hair and nails and animal horn, these cells finally emerge to form the visible skin- the thick, impervious stratum corneum, or "horny layer". Dead skin flakes away from the surface all the time. It is estimated that the epidermis is totally replaced once every three weeks and that an average lifetime sees the shedding of about 40lb (18kg) of dead skin cells.

The underlying dermis, built around a network of protein fibers, gives the skin its elasticity and strength. Loss of elasticity of the dermis is a feature of aging, characterized by folding or wrinkling of the skin. About 0.1 in (3mm) thick, the dermis is interlaced with blood and lymphatic vessels and contains numerous nerve endings, glands, and hair follicles.

Deeper still is the subdermal layer between skin and muscle. Composed of loose connective tissue, this region is heavy with fluid and fat cells that provide insulation, cushion muscles and nerves, and serve as an energy store. It is this subdermal layer of fat (which is thicker in women) that give shape to the body as a whole.

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