The illustration, called INTEGUMENTARY SYSTEM, tells how the functions of the skin assist the other systems of the body (buff color) and how the other systems help the skin carry out these functions (aqua color).

Functions of the Skin

     Skin has a protective function. First and foremost, the skin forms a protective covering over the entire body, safeguarding underlying parts from physical trauma and pathogen invasion. The melanocytes in skin protect it from UV radiation, and the skin’s outer dead cells also help prevent bacterial invasion. The oily secretions from sebaceous glands are acidic, which retards the growth of bacteria. The Langerhans cells in the epidermis phagocytize pathogens and then alert the immune system to their presence.
     Skin helps regulate water loss. Since outer skin cells are dead and keratinized, the skin is waterproof, thereby preventing water loss. The skin’s waterproofing also prevents water from entering the body when the skin is immersed. This function of the skin assists the urinary system, as do the sweat glands, which excrete some urea when sweating occurs.
     Skin produces vitamin D. This function of skin is particularly useful to the digestive and skeletal systems. 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; only a small amount of UV radiation is needed. 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 uptake by the digestive system and both calcium and phosphorus metabolism in cells. Calcium and phosphorus are very important to the proper development and mineralization of the bones. Most milk today is fortified with vitamin D, which helps prevent the occurrence of rickets characterized especially by soft and deformed bones
(Fig. 5.9).
X ray of a child with rickets
Figure 5.9 X ray of a child with rickets. Rickets develops from an improper diet and also from a lack of ultraviolet (UV) light (sunlight). Under these conditions, vitamin D does not form in the skin.
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. As mentioned previously, 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.

Hyperthermia and Hypothermia
Hyperthermia, a body temperature above normal, and hypothermia, a body temperature below normal, indicate that the body’s regulatory mechanisms have been overcome. In heat exhaustion, blood pressure may be low, and salts may have been lost due to profuse sweating. Even so, body temperature remains high. Heat stroke is characterized by an elevated temperature, up to 43°C (110°F), with no sweating. Fever is a special case of hyperthermia that can be brought on by a bacterial infection. When the fever “breaks,” sweating occurs as the normal set point for body temperature returns. At first, hypothermia is characterized by uncontrollable shivering, incoherent speech, and lack of coordination (body temperature 90°-95°F). Then the pulse rate slows, and hallucinations occur as unconsciousness develops (body temperature 80°-90°F). Breathing becomes shallow, and shivering diminishes as rigidity sets in. This degree of hypothermia is associated with a 50% mortality.
     Skin gathers sensory information. The sensory receptors in the dermis specialized for touch, pressure, pain, hot, and cold are associated with the nervous system. These receptors supply the central nervous system with information about the external environment. The fingertips contain the greatest number of touch receptors, allowing the fingers to be used for delicate tasks. The sensory receptors also account for the use of the skin as a means of communication between people. For example, the touch receptors play a major role in sexual arousal, which assists the reproductive system.
     Skin helps regulate body temperature. When muscles contract and ATP is broken down, heat is released. As described in Figure 1.8, the skin, under the direction of the brain, plays an active role in whether this heat is conserved or released to the environment in order to maintain a body temperature of 36.2
° -37.7° C (97° -100°F). If body temperature starts to rise, the blood vessels in the skin, which are a part of the cardiovascular system, 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 carried away as sweat evaporates.
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