REGULATION OF HEAT LOSS. 



861 



2. Through the expired air. This air is warmer than the inspired air, 



and moreover is nearly saturated with water-vapor. The vaporiza- 

 tion of water requires heat, which is, of course, taken from the body 

 supply. Each gram of water requires for its vaporization about 582 

 calories. 



3. By evaporation of the sweat from the skin. The amount lost in this 



way increases naturally with the amount of sweat secreted. 



4. By conduction and especially by radiation of heat from the skin. 



The relative values of these different means of heat loss are 

 estimated as follows by Vierordt: 



1. By urine and feces 1.8 per cent, or 47,500 calories. 



2. By expired air: Warming of air 3.5 " " 84,500 " 



Vaporization of water from lungs 7.2 " " 182,120 " 



3. By evaporation from skin 14.5 " " 364,120 " 



4. By radiation and conduction from skin .73.0 " " 1,791,820 " 



Total daily loss = 2,500,000 " 



It is obvious that the relative importance of these factors will vary 

 with conditions. Thus, high external temperatures will tend to 

 diminish the loss from radiation while increasing that from evapora- 

 tion, owing to the greater production of sweat. The variation 

 in this respect is well illustrated by the following table, compiled 

 by Rubner, from experiments made upon a starving dog:* 



It will be noted that between 25 and 30 C. there was a marked 

 increase in the loss of heat through evaporation. 



In man loss of heat is regulated chiefly by controlling the impor- 

 tant factors of evaporation and radiation. We accomplish this end in 

 part deliberately or voluntarily by the use of appropriate clothing. 

 Clothing of any kind captures a layer of warm and moist air between 

 it and the skin and thus diminishes greatly the loss by evaporation 

 and by radiation. In cold weather the amount and character of the 

 clothing is changed in order to diminish the heat loss. The ideal 

 clothing for this purpose is made of material, such as wool, which, 

 while porous enough to permit adequate ventilation of the air next 

 to the skin, is at the same time a poor conductor of heat and thus 

 diminishes the main factor of loss by radiation. The most impor- 

 tant means of controlling the heat loss, however, is by automatic 

 reflex control through the sweat nerves and the vasomotor nerves. 



* Taken from Lusk, "Elements of the Science of Nutrition," Philadel- 

 phia, 1906. 



