CHANGES IN THE BLOOD BY RESPIRATION. 299 



gases between the blood and the lungs. The blood coming to the lungs 

 comparatively poor in oxygen and charged with carbonic acid, the for- 

 mer gas is absorbed from the air in the pulmonary vesicles, while the 

 latter is discharged at the same time, to be exhaled with the breath. 

 These changes, however, are neither of them complete, but only partial, 

 both for the air and for the blood. The expired air is never deprived 

 of the whole of its oxygen, and contains only about 4 per cent, of its 

 volume of carbonic acid. On the other hand, the venous blood coming 

 to the lungs still contains a moderate percentage of oxygen ; and a cer- 

 tain quantity of carbonic acid is also present in arterial blood. It is 

 only the proportion of -these gases which is changed in respiration, the 

 carbonic acid of the blood being diminished, and its oxygen increased, 

 by its passage through the pulmonary circulation. 



The office of the respiratory apparatus is therefore to afford ingress 

 and egress to the two substances which enter and leave the body in 

 the gaseous form. These two substances have no immediate relation 

 with each other, excepting as to the organ by which they are absorbed 

 and exhaled. They represent the beginning and the end of a series of 

 internal combinations and decompositions, which are among the most 

 essential of the changes contributing to the maintenance of life. 



