Functions of the Skin
Protection
The skin forms a protective covering over the entire body, safeguarding underlying parts from physical traumas. The skin's outer dead cells also help prevent bacterial invasions. The oily secretions from sebaceous glands are acidic, which retards the growth of bacteria. Finally, since outer skin cells are dead and keratinized, the skin is waterproof, thereby preventing fluid (water) loss. The skin's waterproofing also prevents water from entering the body when the skin is immersed.
Skin protects the body from physical trauma, bacterial invasion. and fluid gain or loss.
Synthesis of Vitamin D
When skin cells are exposed to sunlight, the ultraviolet (UV) rays assist them in producing vitamin D. The cells contain a precursor molecule that is converted to vitamin D in the body after UV exposure. Since only a small amount of UV radiation is needed to change the precursor molecule to vitamin D, the skin should not be exposed unnecessarily. Vitamin D leaves the skin and enters the liver and kidneys, where it is converted to a hormone called calcitriol. Calcitriol circulates throughout the body, regulating calcium and phosphorus metabolism where appropriate. Calcium and phosphorus are very important to the proper development and maintenance of the bones. Most milk today is fortified with vitamin D, which helps prevent the occurrence of rickets, or a defective mineralization of the skeleton.
Regulation of Body Temperature
The energy content of nutrient molecules like glucose is converted to ATP energy within mitochondria. When this conversion occurs, heat is released. Only about 40% of the energy available in a glucose molecule becomes ATP energy; the rest escapes as heat. Also, when ATP is broken down, as occurs when muscles contract, heat is released.
One of the best examples of homeostasis is regulation of normal body temperature (36.2-37.7°C [97-100°F]). The hypothalamus, the portion of the brain concerned with homeostasis, is involved in regulating body temperature. The skin also regulates body temperature in two ways: (1) the blood vessels in the dermis can constrict to receive less blood or can dilate to receive more blood; and (2) the sweat glands can remain inactive or can secrete sweat as needed.
Ifbody temperature starts to rise, the blood vessels dilate so that more blood is brought to the surface of the skin for cooling, and the sweat glands become active. Sweat absorbs body heat, and this heat is carred away as sweat evaporates. If the weather is humid, evaporation is hindered, but cooling can be assisted by a cool breeze.
If the outer temperature is cool, the sweat glands remain inactive, and the blood vessels constrict so that less blood is brought to the skin's surface. Whenever the body's temperature falls below normal, the muscles start to contract, causing shivering, which produces heat. The arrector pili muscles attached to hair follicles are also involved in this reaction, and this is why goose bumps occur when a person is cold. If the outside temperature is extremely cold and blood flow to the skin is severely restricted for an extended period, a portion of the skin will die, resulting in frostbite.
The skin is involved in regulation of body temperature. Surface blood vessels dilate and the sweat glands are active when the body is too hot. Surface blood vessels constrict and the svveat glands are inactive vvhen the body is cold.
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