Adaptation to a Hot Environment

When exposed acutely to a hot environment, the hypo-thalamic integrative center acts as described earlier to increase heat loss by causing vasodilation and sweating. Both mechanisms can increase heat output from the body as long as the ambient temperature is below the body core temperature; however, when the ambient temperature is higher, the body is constantly gaining heat by radiation and conduction, and the only means of effecting a net heat loss to match the rate of heat production is by evaporation. Thus, sweating is a critical determinant of the body's response to higher environmental temperatures. When the relative humidity is high, however, the amount of heat that can be lost by sweating is limited by the rate of evaporation; therefore, body core temperature cannot be kept normal over as high a temperature range in a humid environment. The heat that must be lost is

Body heat content

Figure 4 Feedback regulation of body temperature. The integrative center in the hypothalamus compares the body core temperature sensed by the preoptic-anterior hypothalamic thermoreceptors, as modified by input from cutaneous thermoreceptors that sense the environmental (ambient) temperature, with a temperature set-point. It then regulates heat loss and heat production to maintain the body core temperature equal to the set-point. Heat loss from the skin is regulated through control of cutaneous vasomotor activity and sweating. The dashed box surrounds elements involving heat exchange between the skin and the environment. Heat production can be increased by shivering or increased metabolic rate, and decreasing muscle activity can decrease it.

also increased if the body core temperature rises because biochemical reaction rates are accelerated. Metabolic heat production increases about 6% for every °F (10% per °C) of core temperature increase.

When individuals remain for longer periods in a hot climate, there is chronic adaptation in the rate ofsweating over a period of 5 to 10 weeks from the normal maximum of 1.5 L/hr to as much as 4 L/hr; over a period of years in the tropics, individuals actually develop increased numbers of sweat ducts. Large fluid losses due to sweating of this magnitude also require adaptation of body fluid and electrolyte regulation to prevent dehydration, which can lead to heat exhaustion, as described later. Individuals adapt to the large losses of salt and water after several weeks in the tropics by increasing their plasma aldoster-one levels. Aldosterone not only acts on the renal collecting duct to increase Na+ reabsorption (see Chapter 29)

but also increases Na+ reabsorption by the ducts of the eccrine salt glands. Because of the action of aldosterone, the NaCl content of the sweat is reduced. However, the higher aldosterone levels result in greater K+ secretion into both the sweat and the urine, and this can result in dangerous K+ depletion when sweating is severe and dietary K+ intake is low. The K+ loss can be exacerbated when NaCl is ingested without KC1 during periods of heavy sweating.

The body can be subjected to heat stress not only by a hot climate but also by severe exercise in a temperate climate. Body temperature may rise to as high as 39.4 to 40.8°C (103 to 105.5°F) in marathon runners, and this is accompanied by copious sweating and marked cutaneous vasodilation. The loss of salt and water in the sweat as well as the shunting of blood into the skin may result in the heat cramps that often disable athletes. When

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