Copyright © 2007 Dorling Kindersley
The skin, along with hair and nails, provides the body with a protective outer covering that shields it, for example, from harmful solar rays. It also provides our first line of defence against infection, helps control water loss from the body, plays an important role in TEMPERATURE CONTROL, and contains the receptors that provide the sense of touch.
The skin has two main layers, called the epidermis and dermis. The epidermis consists of an upper layer of dead cells and a lower living layer, which replaces cells as they are lost from the upper layer. Beneath the epidermis is the thicker dermis, which overlies an insulating layer of fatty tissue.
Hair grows from follicles, pockets of epidermal tissue that extend down into the dermis. Hair has a cycle of growth, rest, and then loss, when the new hair pushes the old hair out of the follicle. About 100 hairs are lost and replaced in a person’s scalp every day.
The ends of the fingers and toes are covered by nails. These plates of tough protective tissue are made mainly of keratin, a protein also found in hair and skin. Nails grow from a region of living cells called the germinal matrix, which lies underneath a fold of skin called the cuticle.
The blood vessels, hairs, and sweat glands of the skin work together to help control body temperature. If we get too hot, our sweat production increases and blood vessels widen to allow more blood to reach the skin’s surface, where it cools. If we get too cold, these processes go into reverse. In addition, tiny muscles attached to the hair follicles pull the hairs erect, trapping an insulating layer of air next to the skin.