The effectiveness of skin vasoconstriction in reducing heat loss conserving body heat by shunting warm blood into deep veins is influenced by the amount of adipose tissue fat stored under the skin. This means that heat loss is inversely related to the mass of subcutaneous fat; as fat mass increases, heat dissipation decreases. This is due to the fact that fat and skin insulate the body when skin blood vessels are tightly constricted. In fact, fat is an excellent insulator in water and in air figurethat allows Tsk to decrease with a concomitant drop in radiative air and convective air, water heat loss. This explain a why persons with more subcutaneous fat can tolerate a lower water or air temperature than lean persons before shivering begins; and b why metabolic rate is inversely related to body fat content during cold exposure. However, because skin blood vessels pass through, and are distally superficial to, the subcutaneous fat layer see figure these relationships exist only if vasoconstriction is maximal or nearly maximal, because the flow of warm blood is below the level of the fat. When vasoconstriction is mild as in a cool or moderate environment, blood flow to skin is greater and more heat is dissipated to the surrounding air, resulting in a lower core body temperature and a greater demand for heat production via shivering. This also explains why subcutaneous fat is not related to heat storage during exercise in a hot environment. In this case, skin blood flow is great and the insulation afforded by body fat which lies deeper than the skin blood vessels is effectively bypassed.
Anatomical relationship between blood vessels, subcutaneous fat, and skin.
As described in post see Cardiovascular Responses to Heat and Humidity, pagecardiac output liters per minute has two components: stroke volume liters per beat and heart rate beats/minute, both of which are increased in a hot environment. However, a resting exposure to cold air C, F, which increases the demand for blood flow and oxygen delivery to shivering muscle, increases cardiac output solely by an increased stroke volume while heart rate remains constant. Two theoretical mechanisms have been proposed. It is possible that elevated circulating catecholamine levels increase the stroke volume. It also is possible that pressure receptors baroreceptors sense an increase in central blood volume following peripheral vasoconstriction, which depresses heart rate via a reflex. This adaptive response strikingly contrasts with the mechanism employed during exercise in cold air see below. A resting, unclothed exposure to cold air also increases average blood pressure.
All of the adaptive responses shown in figureare under autonomic nervous system control. However, behavior is an important, complex aspect of temperature control in humans and has been named behavioral thermoregulation. Simple examples of this phenomenon, which occurs when a person senses that he or she is cold, include adding items or more layers of clothing, drinking hot liquids, and moving to a warmer location such as indoors. In nonhuman species, behavioral thermoregulation is mostly limited to instincts. The insulative value of clothing will be considered in detail in the final section of this chapter.
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In terms of body fluid balance during cold-air exposure, a detrimental response arises indirectly from skin vasoconstriction reduction of peripheral blood flow and the resulting relative increase of central blood volume. Volume receptors in the left atrium of the heart inhibit the secretion of the hormone AVP see post section titled Urine: An Index of Fluid Balance, page from the pituitary gland in the brain. In the kidneys, the amount of water reabsorbed into the blood is controlled in part by AVP. This means that skin vasoconstriction indirectly results in an increased urine volume during cold exposure. It is difficult to categorize this response as beneficial because it accelerates dehydration, thereby affecting physical performance and health negatively. The impact of dehydration on soldiers and laborers who work in cold environments will be described in the final pages of this chapter.
As with heat and air pollutants, there is evidence to suggest that the human physiological responses to cold-air exposures at rest alter with age. For example, it has been shown, in a comparison of elderly
Exposure to cold triggers an array of adaptive responses designed to maintain core body temperature.
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