1176 PHYSIOLOGY 



violent exercise. Under these conditions a man may sweat profusely when 



the temperature of the surrounding atmosphere is at C. 



The main regulation of heat loss thus takes place by the control of the 

 nervous system over the cutaneous circulation and the sweat-glands. 

 Besides these channels of heat loss, others may play an important part under 

 certain conditions. Heat is lost to the body in warming the food and 

 air which are taken in. It is also lost in respiration in the evaporation of 

 water and the setting free of C0 2 from watery solution into the expired air. 

 The following estimate, by Tigerstedt, represents the proportion of losses 

 in an adult man by these different ways : 



A. WARMING THE FOOD AND AIR 



(1) 1500 g. water drunk at 16 C. and warmed to 37-5 raised therefore Cal. 



22-5 = 33-75 



(2) 1500 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 . = 79-95 



128-70 

 B. Loss OF WATER AND C0 2 IN THE BREATH 



(4) It is assumed that the inspired air is half saturated with watery vapour 



at 15 C. and that the expired air is fully saturated at 37-5 C. Ap- 

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

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

 vapour is 0-537 Cal. ... . =241-70 



(5) The absorption of heat in the liberation of CO 2 from the lungs (800 g.) ; 



0-134 Cal. per g . - 107-20 



348-90 

 From above 128-70 



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'2400 Cal., 

 this sum represents only about 20 per cent, of the total. The remaining 80 

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



If we estimate the total heat loss of an adult man at 2400 calories, we 

 may say that about 5 per cent, of the total heat loss takes place by warming 

 the food and air, about 15 per cent., by the evaporation of water and C0 2 in 

 respiration, and about 80 per cent, by radiation and convection and the 

 evaporation of sweat from the skin. The proportion represented by the 

 last factor will increase very largely in the presence of a high external 

 temperature, or of an excessive heat production in consequence of violent 

 muscular exercise. 



THE NERVOUS MECHANISM FOR HEAT REGULATION 



The accurate balance between heat production and heat loss which is 

 responsible for the nearly constant temperature of man, indicates the active 



