392 TEXT-BOOK OF PHYSIOLOGY. 



of body-heat therefore amounts to 3 X 0.8 X 25 C. = 60 calories 

 = 2.8 per cent. With the removal of the end-products of the 

 foods and drink from the body an equal amount of heat is carried 

 out. 



2. In Warming the Inspired Air. The average temperature of the 



air is 12 C.; the amount of inspired air, about 15 kilograms; 

 the specific heat of air, 0.26. The absorption of body-heat by 

 the air therefore amounts to 15 X 0.26 X 25 = 97.5 = 3.8 

 per cent. The expired air removes from the body a corre- 

 sponding volume. 



3. In the Evaporation of Water ]rom the Lungs. The quantity of 



water evaporated from the lungs may be estimated at 400 grams; 

 as each gram requires for its evaporation 0.582 calorie, the 

 quantity of heat lost by this channel would be 400 X 0.582 = 

 232.8 C. = 9.4 per cent. 



4. In the Evaporation 0} Water jrom the Skin. The quantity of 



water evaporated from the skin may be estimated at 660 grams, 

 causing a loss of heat by this channel of 660 X 0.582 = 384.1 

 C. = 15.3 per cent. 



5. In Radiation and Conduction jrom the Skin. The amount of 



heat lost by this process can be indirectly determined only by 

 subtracting the total amount lost by the above-mentioned 

 channels from the total amount produced. Thus, 2500 774-4 

 = 1725.6 = 69 per cent, would represent the average amount lost 

 by radiation and conduction. 



Regulation of the Mean Temperature. In order that the 

 mean temperature of the body may remain practically constant, the 

 heat produced must be exactly balanced by the heat dissipated. Should 

 there be any want of correspondence between the two processes, there 

 would arise either an increase or a decrease in the mean temperature. 

 As both heat-production and heat-dissipation are variable factors, 

 dependent on a variety of internal and external conditions, their 

 adjustment is accomplished by a complex self-regulating me-chanism 

 involving muscular, vascular, and secretory elements, coordinated by 

 the nerve system. Heat-production varies in intensity and amount, 

 in accordance with a number of conditions, but principally with 

 variations in physiologic activity, the quantity and quality of the 

 food, and changes in the external temperature. All physiologic and 

 especially muscle activity is attended by chemic changes and the 

 evolution of heat. The greater the activity, the larger is the volume 

 of heat. Foods have different physiologic heat values. If the food 

 consumed contains much potential energy and the quantity con- 

 sumed be larger than the average daily requirements, there will be 

 an increase in heat-production. A lowering of the external tem- 

 perature, as in the winter season, leads to increased heat-production 



