The temperature of the surface of the skin is usually several degrees colder than the core of the body; it also varies from one portion of the body to another. The skin and the subdermal fat layers act as an insulating blanket around the core of the body and protect it from rapid heat loss or heat gain. The insulating properties of skin depend on its thickness, which is primarily determined by the thickness of the layer of fat, a good insulator.
The temperature of the skin surface is also determined by the cutaneous blood flow (i.e., the rate of blood flow through the rich capillary network just below the dermis). Cutaneous blood flow is regulated from almost 0 to 30% of the cardiac output. When the cutaneous blood flow is low, the surface temperature of the skin falls, which minimizes heat loss by radiation and conduction. The ability of the skin to reduce these heat losses is limited by the insulating capacity of the skin and the minimal blood flow required to meet the metabolic demands of the tissue. For example, in a very cold climate, the blood flow to the fingers and toes can fall to about 1% of normal. When the cutaneous blood flow is high, the skin temperature can approach the body core temperature, which maximizes the conductive and radiative heat loss.
Cutaneous blood flow is determined by the relative degree of vasoconstriction of precapillary sphincters in the vascular bed just below the dermis. As in other vascular beds, these are under local as well as central regulation (see Chapter 17). Local irritants and heat can cause vasodilation and warming of the immediate skin surface. When a larger area of the skin is warmed, temperature receptors in the skin lead to a reflex increase of blood flow in that area. This response is mediated at the level of the spinal cord. For example, putting your hand in hot water leads to increased blood flow, which is indicated by the redness and relative engorgement of circulation in that hand. Local cooling leads to the opposite reaction.
In response to a generalized change in the temperature of the whole body, the blood flow to the skin is regulated by the hypothalamus. Chilling of the preoptic anterior hypothalamus causes a generalized vasoconstriction in the skin that is mediated by the sympathetic nervous system. Vasoconstriction in the skin is also accompanied by piloerection, the raising of body hair that produces the ''goose flesh'' experienced when one is chilled or frightened. This response is also mediated by the sympathetic nervous system, which produces contraction of smooth muscles associated with the hair follicles. In humans, this response plays only a small role in thermoregulation, but in furry animals, piloerection thickens the insulating layer of the hair and thus reduces heat loss.
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