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The main function of the integumentary system is to provide a protective covering for the body. This system includes the skin and related structures, such as hair, sweat and oil glands, and the nails. It forms a barrier against hot and cold temperatures, harmful chemicals and solar radiation, as well as microorganisms. In addition, it has a major role in the sense of touch and helps maintain the body's temperature. Likewise, the integumentary system is important for vitamin D production and plays a small role in excreting waste.
This system is the body's first defense against harm or injury. The skin protects internal organs and other structures from injuries caused by friction, like scrapes or cuts. It also keeps out microorganisms, like bacteria. Hair insulates the body, protects it from sunlight, and keeps irritants out of the eyes, among other things. Nails prevent injuries to the ends of the fingers and toes.
The skin is also important for maintaining the body's internal temperature. The evaporation of sweat from the skin requires heat, which helps to cool the body. This can be as much as 2 gallons (7.5 liters) or more in extreme conditions. When body temperature rises, blood flow is also increased to the skin, where excess heat is lost through convection. In colder environments, blood flow to the skin decreases, which reduces heat loss.
Providing information about an organism's environment is also a major function of the integumentary system. The nerve endings in the the skin allow the body to sense temperature changes, pressure, and touch. The upper layer contains the receptors responsible for the sense of touch, while those in the lower layer of skin can sense heat and cold, pressure, and pain. Hair follicles, but not the hair itself, respond to touch as well.
Helping the body make vitamin D is an important role of the skin. Part of the process occurs when ultraviolet light from the sun strikes precursor molecules in the skin. The changed molecules are later converted into the active form of vitamin D in the kidneys and liver. The body can produce all the necessary vitamin D in this way, provided enough sunlight is available.
A lesser function of the system is excreting waste. Sweat is nearly all water but contains small amounts of waste products, such as urea, uric acid, and ammonia. Other organs, such as the kidneys, play a much greater role in ridding the body of waste products.