340 



ESSENTIALS OF PHYSIOLOGY. 



being excreted in the urine largely as urea, and to a less extent as 

 other substances, each of which has a calorie value of its own. In 

 determining the physiological calorie value of protein, the heat value of 

 urea and other nitrogenous products must be deducted from the figure 

 5 '6. When this is done, the heat value of protein is reduced to 4*1, 

 which represents its calorie value in the body. The energy set free in 

 the body by the oxidation of the food-stuffs appears partly as heat and 

 partly as muscular work. The energy set free as muscular work may 

 be calculated as heat according to the following equation : 



425 gram-metres of work = 1 calorie. 



Water 



Water 



Air minus CO; and Water; deficient in Oxygen 



Oxygen enters. 



FIG. 128. Diagram to show the principle of the Atwater-Benedict calorimeter. 

 (After Halliburton. ) Starling's Principles of Physiology. 



The heat formed in the body is determined by placing the animal in 

 a suitable calorimeter. Large calorimeters such as that of Atwater and 

 Benedict have been constructed, in which a man can live for two 

 or three days, or longer, and can carry out muscular work. The 

 Atwater-Benedict calorimeter consists of a room, with double, non- 

 conducting walls, containing a series of pipes through which water is 

 flowing at such a rate that the temperature of the room remains 

 constant (fig. 128). The whole of the heat given off by the man 

 warms the water passing through the pipes, and is thus carried off. 

 By measuring the amount of water flowing along the pipes and the rise 

 in its temperature as it passes through the calorimeter, the heat 

 evolved by the individual can be estimated. By means of these 

 methods it is possible to determine, on the one hand, the energy supplied 

 to the body in the food, and, on the other hand, the energy lost from 

 the body as heat and muscular work. It is found that these two 



