608 THE EXIT OF CARBONIC ACID. [BOOK n. 



There are some facts which seem to suggest that the exit of 

 carbonic acid from the blood is assisted by the simultaneous 

 entrance of oxygen, but this is not definitely proved. If such an 

 aid is given, it is probably brought about by the change in the 

 haemoglobin in some indirect way raising the pressure of the 

 carbonic acid in the blood. 



So far then as can be seen at present, both the entrance of 

 oxygen and the exit of carbonic acid by which venous blood is 

 converted into arterial are the simple physical results of the 

 exposure of the blood in the pulmonary capillary to the air of 

 the pulmonary alveoli. 



