LOSS OF HEAT FROM THE BODY 403 



3. LOSS OF HEAT FROM THE BODY 



The heat formed in the body is partly utilized in warming the food, in- 

 cluding water ingested and the air inspired, is partly given off by conduction 

 and radiation through the skin, and partly disappears in the evaporation of 

 water from the air passages and from the skin, and in the liberation of carbon 

 dioxide from the lungs. The following estimate, agreeing essentially with 

 those of^Helmholtz and Rosenthal, indicates approximately the proportion of 

 losses in an adult man by these different avenues. 



A. WARMING THE FOOD AND AIR 



(1) Water drunk at 15 C. and warmed to 37.5 raised therefore 22.5 = 33.75 Cal. 



(2) 1,500 g. food eaten at 25 C. (mean) and warmed to 37.5 raised therefore 



12.5 ; specific heat 0.8 = 15.00 " 



(3) 15,000 g. (= 11,500 1.) air respired at 15 C. and warmed to 37.5 raised 



therefore 22.5 ; specific heat 0.237 = 79.75 " 



128/70 " 

 t B. Loss OF WATER AND C0 a IN THE BREATH 



(4) It is assumed that the inspired air is half saturated with water vapor at 



15 C., and that the expired air is fully saturated at 37.5 C. Approxi- 

 mately 450 g. of water would be given off, therefore, in the form of 

 vapor from the respiratory passages ; the latent heat of the water 

 vapor is 0.537 Cal = 241.70 " 



(5) The absorption of heat in the liberation of CO 3 from the lungs (800 g.) ; 



0.134 Cal. per g = 107.20 " 



34&90 " 



From above 126.70 " 



Total 477.60 " 



The sum of heat losses specified under these five headings amounts to 

 477.60 Cal. Estimating the total heat loss of an adult man at 2,400 Cal., 

 this sum represents only about twenty per cent of the total. The remaining 

 eighty per cent (in round numbers) takes place through the skin. 



The direct calorimetric measurements by Atwater give approximately the 

 same result. In a series of experiments lasting forty days the mean heat loss, 

 in the case of a resting man, by conduction and radiation was 1,669 Cal., through 

 the urine and faeces 31 Cal., by evaporation of water 550 Cal. i. e., in percent- 

 ages 74.2, 1.4, and 24.4 respectively. In the case of a man at work, the mean 

 for twenty days by conduction and radiation was 2,277 Cal., through urine and 

 faeces 19, by evaporation of water 1,126; or in percentages, 66.5, 0.6, and 32.9 

 respectively. The proportion of evaporation from the skin and from the lungs 

 was not determined. 



The skin gives off heat to the surrounding air, or to other cold substances 

 coming in contact with the body, by conduction, radiation and evaporation. 

 The relative importance of these factors varies greatly under different circum- 

 stances. The quantity of water vapor given off from the skin depends to a 

 great extent both upon the temperature and humidity of the surrounding air, 

 and also upon the heat production going on in the body, which in turn varies 

 with the kind and the quantity of food eaten and the amount of work done. 



