The human integumentary system is made up of the skin, hair, nails, and glands, and it 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 also helps to regulate body temperature. If heat builds up in the body, sweat glands in the skin produce 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 at the body surface; heat is, thus, conserved. In addition to temperature regulation, the...