294 



TEMPERATURE AND HUMIDITY 



above the central zone. If a single point in 

 the middle of the range is considered, the 

 addition of normal clothing permits a heat 

 balance near 22°C which is similar in comfort 

 and body temperature values to that seen 

 in nude subjects near 28°C. 



Reactions of the Body to a Cold Environment 



In the Zone of Body Cooling, the primary 

 reaction of the body is a lowering of skin 

 temperature due to constriction of the super- 

 ficial blood vessels of the skin. Under con- 

 ditions of long exposure to a chiUing atmos- 

 phere, there would be, as noted above, a 

 compensatory rise in metabolism, generally 

 associated with shivering and with a tend- 

 ency to move about. 



TABLE VIII 



Average Thermal Conductance of Tissues 

 FOR THE Nude Subject Above and Below 30°C 



Temperature °C 



Above 30°. 

 Below 30°. 



Conductance* K 



Mini- 

 mum 



18.0 

 10.6 



Mean Maximum 



20.3 

 12.6 



23.8 

 15.9 



* In kg.cal./ni.Vhr./°C. 



The influence of the fall in skin tempera- 

 ture upon heat loss may be analyzed in terms 

 of a factor referred to as skin conductance, 

 which is included in Fig. 6. The total heat 

 flow from the interior of the body to its sur- 

 face is equal to the difference between me- 

 tabolism and positive heat change, or to the 

 sum of metabolism and negative heat 

 change. If we divide this figure by the 

 surface area of the body and divide again by 

 the difference between rectal temperature 

 and skin temperature, we have a measure of 

 heat flux per unit area of skin. In other 

 words. 



(14) "i^^iT. 



- Ts) = K. 



The value of conductance {K), determined 

 in this way, shows (for the same nude sub- 



ject for whom data are presented in Fig. 6) 

 the differences, above and below the critical 

 temperature given in Table VIII. 



The conductance, as thus recorded, obvi- 

 ously depends on two major factors, thermal 

 conductance of the body tissues, and the flow 

 of blood which brings warm blood to the 

 surface to be cooled. The first of these fac- 

 tors is influenced, however, not only by the 

 specific conductivity of the flesh itself but 

 also by the depth of surface tissues that are 

 actually cooled below a normal value. This 

 gradient is itself obviously influenced to a 

 considerable extent by blood flow, but is also 

 affected by the progressive chilling of deeper 

 and deeper layers of body tissue. The fact 

 that the rate of lowering of skin temperature 

 (and hence of conductance) at low operative 

 temperatures is so much more rapid than the 

 rate of rise in skin temperature at high opera- 

 tive temperatures indicates clearly the im- 

 portance of vasoconstriction; and the sud- 

 denness with which the picture changes near 

 30°C for the nude subject suggests a definite 

 reflex response. 



It should be emphasized that quantitative 

 values may vary widely in different subjects. 

 While a very stout subject showed a con- 

 ductance of 18.3 under warm conditions and 

 a conductance of 12.4 under cold conditions, 

 the corresponding values for a less well- 

 insulated slender subject were 25.0 and 13.7. 

 The second subject, too, showed very high 

 values {K over 35) at extremely high opera- 

 tive temperatures. In both cases, however, 

 there was the same sharp break near 30°C 

 operative temperature, indicative of vaso- 

 constriction. 



The physiological reactions in this process 

 have been reviewed in some detail elsewhere 

 (17) with reference to the extensive studies 

 of Kleiber, Burton, Bazett, Hardy, and 

 DuBois. All that need be said further here 

 is that below an operative temperature of 

 28°C, for both nude and clothed subjects, the 

 value of K lies approximately constant be- 

 tween 10 and 12 (with gradients between 

 rectal and skin temperature varying from 



