METABOLISM. 



341 



balance one another within the limits of experimental error, and that 

 the energy lost from the body has its origin entirely in the potential 

 energy taken into the body in the food. Thus the principle of the 

 conservation of energy is as true for living beings as it is in the rest of 

 the organic and the inorganic world. 



THE RESPIRATORY QUOTIENT AND EXCHANGE. 



The respiratory exchange is the total quantity of oxygen taken into 

 the body and of carbonic acid discharged from the body in a given time. 



The respiratory quotient, as already mentioned (p. 250), is the 

 ratio of the amount of carbonic acid discharged from the body to the 

 amount of oxygen taken in in a given time. 



In man the amount of oxygen used and of carbonic acid evolved in 

 the metabolic changes taking place in the body under varying conditions, 

 such as rest or muscular exercise, can be determined by means of the 

 calorimeter just described. The air leaving the calorimeter passes 

 through vessels containing soda lime, which absorbs carbonic acid, the 

 increase in weight of these vessels giving the weight of carbonic acid 

 exhaled in a given time. The oxygen used by the individual is replaced 

 from a cylinder, the amount supplied being measured. A continuous 

 circulation of air through the chamber is provided by a small pump. 



The respiratory quotient and 'exchange may also be determined 

 approximately by finding with the aid of a spirometer the average 

 volume of air breathed in a minute, and by analysing a sample of 

 expired air. Thus, if a man .breathes 500 c.c. of air at each breath and 

 his respirations are 16 per minute, he breathes 8 litres per minute. 

 Assuming that the expired air contains 16 per cent, oxygen and 4 per 

 cent, carbonic acid, he must have absorbed 5 c.c. oxygen from each 

 100 c.c. of air breathed, namely 400 c.c. ; similarly, he must have 

 breathed out 320 c.c. carbonic acid, and the respiratory quotient is 

 320 

 400 



For small animals the apparatus devised by Haldane and Pembrey, 



