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Biology The Skin

The skin is the largest organ of the body. The skin, including nails, hair, and sweat glands, is sometimes called the integumentary system. If the skin of a 150-pound (68-kilogram) person were spread out flat, it would cover approximately 20 square feet (1.9 square meters). Skin has two layers: the epidermis and the dermis. Subcutaneous tissueprovide protection for the skin.
The epidermis forms the outermost layer of the skin. It serves as a barrier between the outside world and the inner tissues of the body. The outer portion of the epidermis consists of tough, dead cells that prevent bacteria, chemicals, and other harmful substances from entering the body. It also protects the body’s inner tissues from the harsh rays of the sun and prevents the loss of water from these tissues.
The dermis is the lower layer of the skin. The dermis helps keep the temperature of the body within its normal range. The body produces tremendous amounts of heat as it uses food. Some of this heat escapes from the body through the blood vessels in the dermis. When the body needs to retain heat, these blood vessels narrow and so limit heat loss. When the body needs to give off heat, the blood vessels in the dermis expand and so increase heat loss. The sweat glands, which come from the epidermis, also help control body temperature. These glands produce sweat, which is released through pores on the skin surface. As the sweat evaporates from the surface, it cools the body.
The dermis also serves as an important sense organ. Nerve endings within the dermis respond to cold, heat, pain, pressure, and touch.
Subcutaneous tissues lie directly beneath the skin. They provide extra fuel for the body. The fuel is stored in fat cells. Subcutaneous tissues also help retain body heat and cushion the inner tissues against blows to the body.

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