METABOLISM. 343 



Further, the respiratory exchange varies with the size of the animal. 

 The smaller the animal the greater is its surface relatively to its 

 weight, and since the body loses heat chiefly from its surface, the relative 

 loss of heat must be greater in a small than in a large animal. In order 

 to make up for this loss the smaller animal must produce a relatively 

 larger amount of heat, and must use up in this process relatively more 

 oxygen, than a bigger animal. . It is found, in fact, that metabolism 

 is more active and the respiratory exchange is greater, weight for 

 weight, in small animals than in large ones. 



Apart from the question of size, the consumption of oxygen is 

 also relatively greater in growing than in adult animals, since oxygen 

 is being used not only in the chemical changes necessary to maintain 

 the weight of the animal, but also in the metabolic processes associated 

 with growth. 



The Respiratory Quotient. The size of the respiratory quotient 

 is determined almost .entirely by the character of the food consumed by 

 an animal. If an animal were living solely upon carbohydrate food, all 

 the oxygen taken into the body would reappear as carbonic acid, and 

 the quotient would be 1, in accordance with the following equation : 



. . 



oO. 2 



If the diet consisted entirely of fat, the oxygen taken into the body 

 would not all reappear as carbonic acid ; some of it would be vised in 

 converting hydrogen into water, and the quotient would be less than 

 1, the oxidation being represented thus : 



C 3 H 5 (C 18 H 83 ) 3 + 800., = 57CO., + 52H,0. ^9* = 0-71. 



oUU 

 Olein. 



On a purely protein diet part of the oxygen would be combined 

 with nitrogen and sulphur, as well as with hydrogen, and the respiratory 

 quotient would be approximately 0'8. In man living on a mixed diet 

 the quotient is usually about 0'85, and can be lowered by the exclusion 

 of carbohydrate, or raised when the diet consists mainly or exclusively 

 of carbohydrate food. The respiratory quotient is thus of great value, 

 in that it gives an indication of the nature of the food which is being 

 oxidised in the body under various conditions. During muscular 

 exercise, for example, the quotient rises very slightly, and it is evident 

 that the active muscle uses a rather larger proportion of carbohydrate 

 than resting muscle. In a large number of experiments made on the 

 same individual, the average respiratory quotient was O85 during rest, 

 and 0'88 during muscular exercise. 



