REGULATION OF HEAT LOSS. 953 



This amount is increased greatly under conditions demanding 

 much muscular work. This loss of heat is, of course, made good 

 by the production of an equal amount within the body by the oxi- 

 dation of the food material.* Actual experiments upon different 

 animals show that small animals produce more heat in proportion 

 to their weight than larger animals of the same species, owing to 

 their relatively larger surface, and, therefore, greater loss of heat. 

 This fact has been expressed by Rubner in what he calls his sur- 

 face area law. According to this law the metabolism is pro- 

 portional to the surface area, or for the same amount of surface 

 area there will be the same production of heat. He estimates 

 that in man there is produced in twenty-four hours for each 

 square meter of surface 1042 calories. More extensive observa- 

 tions by later observers give a lower figure, 700 to 900 calories. 



HEAT REGULATION. 



From a general standpoint the most important problem that the 

 physiologist has to study is the means by which the heat production 

 and heat loss are so regulated as to maintain a practically constant 

 body temperature. Experiments show that the mechanism of 

 heat regulation is very complex and is two-sided, that is, the body 

 possesses means of controlling the loss of heat as well as the produc- 

 tion of heat, and under the conditions of normal life both means 

 are used. 



Regulation of the Heat Loss. Heat is regularly lost from our 

 bodies in a number of different ways, which may be classified as 

 follows : 



1. Through the excreta, urine, feces, saliva, which are at the temperature 



of the body when voided. 



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 0.5 cal. 



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 48 calories. 



2. By expired air: Warming of air 3.5 



Vaporization of water from lungs 7.2 



3. By evaporation from skin 14.5 



4. By radiation and conduction from skin 73.0- 1792 



Total daily loss = 2470 " 



*See Rubner, "Zeitschrift f. Biologie," 19, 535, 1883; and "Gesetze 

 des Energieverbrauchs," 1902. 



