ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



291 



the zero line and heat loss helow the zero line 

 in kg.cal. per man per hour. Each abscissa 

 temperature, if followed upward, will be 

 found to cut four trend lines, and the values 

 on the four trend Unes thus intersected repre- 

 sent the average values for the elements of 

 heat loss at that operative temperature. 



The upper line of the graph (crossed cir- 

 cles) indicates observed metabolism which, 

 in this case (that of a very stout subject 

 weighing 230 pounds) showed a very slight 

 tendency to increase with rising environmen- 

 tal temperatures. A minimum figure of 87 

 Calories was observed at operative tem- 

 perature of 18.5°C, and a maximum value 

 of 106 Calories at a temperature of 28.5°C; 

 both were exceptional. All other values 

 were between 91 and 104 Calories. 



The open circles represent the heat inter- 

 change by convection and radiation com- 

 bined, since the abscissa is plotted on the 

 basis of operative temperature which ex- 

 pressed the combined effect of both proc- 

 esses. It will be noted that heat gain, due to 

 these factors (above an operative tempera- 

 ture of 35°C) shows a straight line relation- 

 ship to operative temperature. Below 35°, 

 however, heat loss due to conduction plus 

 radiation does not fall off as rapidly as opera- 

 tive temperature. The change in slope is 

 due to physiological adaptations in the form 

 of a decrease in skin temperature, which will 

 be discussed in a succeeding paragraph. 



The curve for evaporative heat loss (solid 

 circles) shows a gradual increase in the cold 

 zone, from 22 Calories at 18.5° and 23.5°C 

 to 33 Calories at 29.5°C. Above 33°, evapo- 

 rative heat loss increases very rapidly — to 61 

 at 33.5° and to 192 at 41.0°C. The slight 

 increase in evaporative cooling between 18° 

 and 30°C is due to the physical factor of 

 changes in atmospheric vapor tension which 

 produces greater evaporation at higher air 

 temperatures. The rapid increase in evapo- 

 ration above 30°C is due to increased sweat 

 secretion, which will be analyzed in a subse- 

 quent paragraph. 



Heat exchange is positive (the mean tem- 



perature of body tissues is rising) at opera- 

 tive temperatures above 30°C, except in one 

 single instance (with very high wall tem- 

 perature), where a negative change of 9 

 Calories was recorded. The positive values 

 varied from 4 to 22 Calories, and the fact 

 that the higher figures appear between 30° 

 and 36°C (with low figures above 36°), may 

 be coincidental or may be due to the very 

 effective evaporative cooling at high tem- 

 peratures. At operative temperatures below 

 30°C, values for heat change were, except in 

 a single instance, negative (cooling of body 

 tissues). With minor deviations, the in- 

 crease in negative heat change is steady from 

 6 Calories at 29.5°C to 123 Calories and 110 

 Calories at 18.5°C. 



Considering the data of Fig. 5 as a whole, 

 we note three major zones of heat regulation 

 which may be discussed separately. 



Zone of Vasomotor Control and Unstressed 

 Evaporative Regulation {Comfort Zone) 



At operative temperatures between 28° 

 and 33°C (with the particular nude subject 

 in semi-reclining position) there is a zone of 

 essential thermal equilibrium in which vaso- 

 motor regulation and unstressed evaporation 

 are characteristic. This unconscious regula- 

 tion is brought about by minimal sensations 

 from the skin, inducing changes in moisture 

 on the skin and quantity of blood flowing to 

 the skin, in such a way that a perfect heat 

 balance is maintained. In environments 

 either hotter or cooler the large volume of 

 sensation due to stimulation of the tempera- 

 ture receptors of the skin is undoubtedly the 

 source of much discomfort and distraction, 

 thereby causing a reduction in the over-all 

 efficiency of the man. The mean of 36 indi- 

 vidual experiments in the comfort zone 

 shows a metabolism of 95 Calories, a heat 

 loss by evaporation of 42 Calories, and a 

 positive heat change of 7 Calories. This 

 state of relative equilibrium is, therefore, 

 characterized by a moderate ratio of evapo- 

 rative cooling (47 percent of total heat loss), 

 indicating a substantial acceleration of sweat 



