Integumentary System

The human integumentary system is made up of the skin, hair, nails, and glands, and serves many protective functions for the body. It prevents excessive water loss, keeps out microorganisms that could cause illness, and protects the underlying tissues from mechanical damage. Pigments in the skin called melanin absorb and reflect the sun's harmful ultraviolet radiation. The skin helps to regulate the body temperature. If heat builds up in the body, sweat glands in the skin produce more sweat which evaporates and cools the skin. In addition, when the body overheats, blood vessels in the skin expand and bring more blood to the surface, which allows body heat to be lost. If the body is too cold, on the other hand, the blood vessels in the skin contract, resulting in less blood is at the body surface, and heat is conserved. In addition to temperature regulation, the skin serves as a minor excretory organ, since sweat removes small amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since it contains millions of nerve endings that detect touch, heat, cold, pain, and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.

In an adult, the skin covers about 21.5 sq ft (2 sq m), and weighs about 11 lb (5 kg). Depending on location, the skin ranges from 0.02-0.16 in (0.5-4.0 mm) thick. Its two principal parts are the outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is found below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.