skin

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Covering of the body of a vertebrate. In mammals, the outer layer (epidermis) is dead and its cells are constantly being rubbed away and replaced from below; it helps to protect the body from infection and to prevent dehydration. The lower layer (dermis) contains blood vessels, nerves, hair roots, and sweat and sebaceous glands (producing oil), and is supported by a network of fibrous and elastic cells. The medical speciality concerned with skin diseases is called dermatology.

The skin helps to protect the body from drying out. It is waterproof and covered with dry, dead cells, so little water is lost from skin cells. However, water is lost from a human body when the body sweats. The skin helps to regulate body temperature. Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre has special cells sensitive to the temperature of blood flowing through the brain (receptors). Temperature receptors in the skin also send nerve impulses to this centre giving information about skin temperature.

If the body temperature is too high blood vessels supplying the skin capillaries expand (dilate) so that more blood flows through the capillaries and more heat is lost. To further assist heat loss sweat glands release sweat, which cools the body as it evaporates.

If the body temperature is too low blood vessels supplying the skin capillaries constrict to reduce the flow of blood through the capillaries.

Other than measuring the temperature, the skin senses the environment in several other ways. Some receptors in the skin are sensitive to touch and pressure. If they are stimulated, nerve impulses are sent to the brain carrying information. It also protects the body from disease-causing organisms, which find it hard to penetrate the skin.

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