CHAP, ii.] RESPIRATION. 577 



ought to be found to be equal to, and not more than equal to, the 

 carbonic acid pressure of the venous blood of the pulmonary artery. 

 And this is the result which has been arrived at ; it has been 

 found that the pressures of the carbonic acid of the occluded air 

 and of the venous blood of the right side of the heart are just 

 about equal. Hence the evidence so far as it goes is distinctly in 

 favour of the view that the escape of carbonic acid from the blood 

 into the pulmonary alveoli is simply due to diffusion, and that 

 there is no need to seek for any further explanation. There is, 

 as far as we can see at present at all events, no necessity, any 

 more than in the case of oxygen, to suppose that the wall of 

 the pulmonary alveoli has any specific secretory power of dis- 

 charging carbonic acid from the blood independently of or in 

 antagonism to the influence of pressures, or that it exerts any 

 special influence at all as a diffusion septum. 



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. 



As 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. 



F. 37 



