ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



297 



up to a mean sldn temperature of 32°C. 

 Above this point it rises progressively to over 

 90 Calories per square meter at a skin tem- 

 perature of over 35°C. Thus, while evapo- 

 ration remains relatively constant in the 

 Zone of Body Coohng as skin temperature 

 falls, the reverse is the case in the Zone of 

 Evaporative Regulation. 



In analyzing the process of evaporative 

 regulation, it has been convenient to intro- 

 duce a new physiological concept, that of 

 wetted area (15) . This factor is discussed in 

 the section on Physical Processes under 

 Evaporation . The utility of the wetted area 

 concept is that it permits an estimate of the 

 maximum value of evaporation under stress 

 conditions, and affords a reasonable explana- 

 tion of the independence of relative humidity 

 and unstressed evaporative heat loss. A 

 further discussion would not be valuable 

 here, except to note that it has been a useful 

 concept in developing the practical values 

 for the upper limits of evaporative regu- 

 lation. 



The Upper Limits of Evaporative Regulation 



Precise and effective as the process of 

 evaporative regulation is, it operates only 

 within certain defined limits. The most im- 

 portant limiting factor is the amount of 

 moisture that the atmosphere can actually 

 absorb from a saturated surface at skin tem- 

 perature (35-36°C). If we assume a maxi- 

 mum wetted area and a skin temperature of 

 35.6°, we can compute for any combination 

 of atmospheric temperature and humidity 

 the number of Calories per unit area of skin 

 which will be evaporated. ICn owing the 

 constants for convection and radiation, as 

 determined by methods outlined previously, 

 and assuming a metabolic rate of 47 

 kg.cal./m.yhr., we can compute the amount 

 of heat which must be given off by evapora- 

 tion to maintain equilibrium. Any com- 

 bination of atmospheric temperature and 

 humidity for which the possible heat loss by 

 evaporation does not equal the evaporative 

 heat loss necessary for equilibrium lies be- 



yond the upper limit of tolerance. With 

 saturated air at any temperature near skin 

 temperature, evaporation is, of course, 

 nearly zero, in accordance with the very 

 small differential in vapor pressure repre- 

 sented by the difference in the saturation 

 values for air and mean skin temperatures. 



By the use of this technique of estimating 

 evaporative limits, it has been possible to 

 compute the upper hmits of evaporative 

 regulation given in Fig. 8. 



The accuracy of such limits is dependent 

 upon the validity of the constants used, 

 which in this case are drawn from the study 

 by Gagge (15). For conditions of low air 

 movement, the values are probably quite 

 reliable. The effects of added air movement 

 on wetted area are less well estabhshed and 

 the limits noted may be offered as best ap- 

 proximations on the basis of presently avail- 

 able data. The values for increased air 

 movement assume that air movement affects 

 evaporation in a manner analogous to the 

 effect of increased air movement on convec- 

 tion loss. 



In a very hot and dry atmosphere, such as 

 one finds in some desert regions, another 

 factor comes into play: the physiological 

 capacity of the body to produce sweat. In 

 such an atmosphere, the difference of vapor 

 pressure between skin and air is so great that, 

 even with maximum vasodilation, the sweat 

 glands are unable to maintain a high wetted 

 area. However, under sub-surface marine 

 conditions this is not the typical case. The 

 three curves of Fig. 8 indicate the upper 

 limits of tolerance fixed by the evaporative 

 power of the atmosphere for nude subjects 

 under three conditions. Curve A is for a 

 subject at work (with a total heat produc- 

 tion of 425 Calories per hour and minimal air 

 movement) ; B is for a subject at rest with a 

 total metabolism of 85 Calories at minimal 

 air movement; C is for a resting subject ex- 

 posed to air movement at a rate of 100 feet 

 per minute. The crosses on Curves A and C 

 indicate the upper limit of tolerance which 

 would be set by the physiological capacity of 



