EESPIRATION. 263 



that all the oxygen which goes into the blood in the lungs dees 

 so by passing through the stage of dissolved oxygen in the 

 plasma. 



At the same time the air in the lungs becomes saturated with 

 aqueous vapour at the temperature of the body. 



The number of respirations per minute varies with the age, 

 c., of the animal. For adult animals the following is the 

 average : 



Horse ... 912 



Ox ... 1518 



Sheep ... 1316 



The change produced in the composition of air by respiration 

 is indicated in the following table, which gives the average 

 composition of air before and after inspiration : 



Inspired air. Expired air. 



Oxygen ... 21-00 16-50 



Nitrogen ... 78'03 78-03 



Carbon dioxide ... -03 4-43 



Argon, neon, &c.... 0-97 0*97 



Water vapour ... variable saturated 



Temperature ... variable about 36 C. 



The most important changes undergone by the air are the loss 

 of oxygen and the gain in carbon dioxide and aqueous vapour. 

 Oxygen by its union with carbon gives rise to its own volume 

 of carbon dioxide. The increase in the volume of carbon 

 dioxide during respiration is, however, almost always less than 



the decrease in that of oxygen. The quotient ~ 2 , ? 



0. 2 absorbed 



is known as the respiratory quotient. It varies with circum- 

 stances, particularly the food and the amount of muscular 

 exertion. It is usually about -9. The disappearance of oxygen 

 is probably due to its being used to oxidise hydrogen to water 

 (say from fats and proteids in the food), and also in the forma- 

 tion of certain waste products, e.g., urea. 



Seat of oxidation in the body. Although almost all the 

 oxygen absorption of the body occurs in the lungs (a small 

 amount also through the skin), the act of union with the 



