296 



TEMPERATUEE AND HUMIDITY 



in the usual semi-reclining position. It will 

 be noted that in the Zone of Evaporative 

 Regulation (at the right of the chart) values 

 for the trunk remain constant (between 34.9° 

 and 35.2°C), and values for the upper ex- 

 tremities also remain constant (34.6°- 

 34.7°C). Values for the head rise slightly 

 with rising environmental temperature in 

 this Zone (from 34.8°C at operative tem- 

 perature 32.2° to 35.7° at operative tempera- 

 ture 38.9°C). The temperature of the legs 

 also rises from 34.4° at operative tempera- 

 ture of 33.3° to 35.1 °C at operative tempera- 

 ture of 38. 9°C. 



Below 33°C operative temperature for the 

 lower extremities, and below 32°C operative 

 temperature for the other three areas, all 

 skin temperatures begin to fall more rapidly 

 as operative temperatures fall (passing to- 

 ward the left of the chart) . The rate of fall 

 is least rapid with the trunk and most rapid 

 with the lower extremities. 



Physiological Adaptations in the Zone of 

 Evaporative Regulation 



In this region, which is above 33°C for the 

 nude and 29°C for the clothed individual, the 

 body has an extraordinarily effective process 

 of compensation for an environment of high 

 heat stress, a compensation which, in its 

 physical manifestations, is very simple, being 

 accomphshed by a progressive increase in 

 evaporation. The perfection of this reac- 

 tion, however, involves physiological adjust- 

 ments of considerable complexity. 



Under the conditions of ordinary studies 

 of basal metabohsm, with reasonably stand- 

 ard conditions of thermal environment, the 

 evaporative heat loss from the body is mod- 

 erately low and highly constant. DuBois 

 (13) tabulates a large number of experiments 

 of this type for normal persons and those 

 suffering from various diseases, in which the 

 percentage of metabohc heat lost by evapo- 

 ration varies only between 21 percent and 28 

 percent, averaging about 25 percent. Under 

 such conditions, the insensible perspiration 

 bears a direct straight-line relationship to 



metabohsm. Under hot environmental con- 

 ditions, on the other hand, the evaporative 

 heat loss may rise, as we have seen, to very 

 high values. The obvious environmental 

 factor which is related to the phenomenon of 

 increased sweat secretion is air temperature; 

 but this relationship is only manifest above 

 the critical level of the area of thermal equi- 

 librium. Below an operative temperature 

 of about 30°C, with the nude semi-reclining 

 subject, evaporative heat loss is shght. 

 Above this point comes a sharp increase. 

 This increase in evaporation is, of course, 

 generally proportionate to progressive in- 

 crease in air and wall temperature; but it is 

 not governed by physical moisture demands 



TABLE IX 



Amount of Water that Can Be Retained bt 



Air at Various Temperatures 



of the atmosphere, since it bears no relation 

 to the relative humidity of the atmosphere 

 in this range. The physical capacity of the 

 air to take up moisture changes enormously 

 at various humidities, as indicated in 

 Table IX. 



Air which is completely saturated at 0°C 

 is only 28 percent saturated when warmed 

 to 20°C. Yet, in spite of wide variation in 

 temperature and relative humidity, the ac- 

 tual evaporation from the body over a very 

 considerable thermal range is maintained at 

 equilibrium level by a varying rate of secre- 

 tion of sweat on the body surface in accord 

 with specific adaptive physiological proc- 

 esses (43). 



The best index of the rate of evaporative 

 heat loss is to be found in skin temperature. 

 Evaporative heat loss remains fairly con- 

 stant (below 20 Calories per square meter) 



