IV THE GASEOUS INTERCHANGE 153 



quantity of carbonic acid in the body is reduced, and 

 indeed the whole condition is one which cannot be 

 maintained for more than a few minutes. 



The activity of the respiratory process is greatly 

 modified by the circumstances in which the body is placed. 

 Thus, cold greatly increases the quantity of air which 

 is breathed, the quantity of oxygen absorbed, and of 

 carbonic acid expelled : exercise and the taking of food 

 have a corresponding eflfect. 



In proportion to the weight of the body, the activity of 

 the respiratory process is far greatest in children, and 

 diminishes gradually with age. 



The excretion of carbonic acid is greatest during the 

 day, and gradually sinks at night, attaining its minimum 

 about midnight, or a little after. 



The quantity of oxygen which disappears in proportion 

 to the carbonic acid given out, is greatest in carnivorous, 

 least in herbivorous animals — greater in a man living on 

 a flesh diet, than when the same man is feeding on vege- 

 table matters. 



9. The Nature of the Respiratory Changes in the 

 Lungs and Tissues. — The essential difference between 

 venous and arterial blood is, as we have previously ex- 

 plained, entirely dependent upon the relative amounts of 

 the two gases, oxygen and carbonic acid, which they re- 

 spectively contain. We have also pointed out where the 

 changes from arterial to venous blood and vice versa, take 

 place, and have indicated the general causes of the con- 

 version as being an interchange between the blood and the 

 tissues on the one hand, and between the blood and 

 the stationary air in the lungs on the other. But we 

 have not so far dealt with the nature of the processes 

 involved in effecting the interchange, and to these we 

 must now turn our attention. 



A clear understanding of certain facts and principles 



