13章英文.docVIP

13.Temperature Regulation CHAPTER 13 TEMPERATURE REGULATION The protected human being can tolerate variations in environmental temperature between -50℃ and 100℃. But person can tolerate a variation of only 4℃ in deep body temperature without impairment of optimal physical and mental performance. Changes in body temperature affect cellular structures, enzyme systems, and numerous temperatures-dependent chemical reactions and physical processes that take place in the body. The maximal limits that the living cell can tolerate range from about -1℃ at one end of the scale, when the ice crystals formed during freezing break the cell apart, to thermal heat coagulation of vital proteins in the cell at about 45℃ at the other end of the scale. Only for shorter periods of time can cells tolerate an internal temperature exceeding 41℃. In fact, many animals, including humans, live their entire lives only a few degrees removed from their thermal death point. ?????? The hot end of the scale is more problematic than the old end, because people can protect themselves more easily against overcooling than overheating. Consequently, the controlling mechanism for temperature regulation is particularly geared to protect the body tissues against overheating (Hardy 1967). ????? The purpose of this chapter is to describe the different factors that help regulate body temperature; to explain the effects of climate, physical activity, and acclimatization; and to discuss the limits of tolerance to heat and cold. Heat Balance If the heat content of the body is to remain constant, heat production and heat gain must equal heat loss, according to the following equation: M?R?C-E=0, where M= metabolic heat production; R= radiant heat exchange (positive if the environment is hotter than the skin temperature, but negative if the temperature of the environment is lower than that of the skin), C= convective heat exchange (positive if the air temperature is higher than that of the skin, negative if the