ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



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295 



4.5° up to 9.5°C, as one proceeds to cooler 

 and cooler environments). At operative 

 temperatures over 30°C, the K values rise 

 more or less steadily with higher operative 

 temperatures, reaching 23.8 for the nude and 

 38.7 for the clothed subjects at an operative 

 temperature of 38.9°C. In this area the 

 great increase in conductance is accompanied 

 by progressive reduction in the gradient be- 

 tween rectal and skin temperature (4.5°- 

 1.5°C). Clearly, in the Zone of Body Cool- 

 ing, a maximum vasoconstriction has been 

 reached, and K remains essentially constant. 



layers of the body equal to 21 percent of the 

 normal specific output of the heart. 



The lowering of conductance by vasocon- 

 striction in cold environments, together with 

 the lowering of skin temperature which ac- 

 companies it, is a useful adaptative mech- 

 anism for reducing the strain put upon 

 the body by such environments. That 

 this mechanism is quite unable to cope 

 effectively with any considerable fall in oper- 

 ative temperature is clear, however, from the 

 rapid increase in negative heat change that 

 is noted below 28°C operative temperature 



21 23 25 27 29 31 33 



OPERATIVE TEMPERATURE IN *C 



35 



37 



39 



Fig. 7. Mean values for the segmental skin temperatures of four unclothed subjects at operative 

 temperatures from 20° to 39° C. T, H, U, L, represent Trunk, Head, Upper, and Lower Extremities. 



In the Zone of Evaporative Regulation, on 

 the other hand, the relatively rapid rise of 

 conductance must be due chiefly to progres- 

 sively increasing blood flow. Analysis of 

 the data indicates that, under the critical 

 conditions where there is a minimum K value 

 and a difference of 4.5°C between skin and 

 body temperature, the depth of the tissue 

 layer cooled below the general body tempera- 

 ture is of the order of 2.2 cm. The increase 

 in blood flow corresponding to the conduct- 

 ance of 38.9 observed at 39°C operative 

 temperature for the clothed subject indicates 

 an increase in blood volume in the peripheral 



with the nude subject, and below 20°C with 

 the clothed subject. Clearly, such marked 

 cooling of body tissues could not continue 

 progressively for any extended period 

 without serious interference with bodily 

 functions. 



Actual Skin Temperature for Various 



Regions of the Body Surface Beyond 



the Area of Equilibrium 



Fig. 7 shows the difference in skin tem- 

 perature for four of the main regions of the 

 body at 20 different operative temperatures, 

 using the mean values for four nude subjects 



