THE GASES OF THE BLOOD 261 



In order to answer this question it is necessary to know the partial 

 pressures of oxygen and carbon dioxide in the alveoli. The per- 

 centage of oxygen or carbon dioxide in expired air cannot tell us 

 the pressure of the gas in the alveoli, for the air in the upper part 

 of the respiratory tract is necessarily expelled along with the 

 alveolar air, and alters the proportions. But the mean of the oxygen 

 or carbon dioxide percentages in samples taken from the last por- 

 tions of the air of two deep expirations, one following an ordinary 

 inspiration and the other following an ordinary expiration, is the 

 mean percentage in the alveoli. The average percentage of oxygen 

 may be taken as 14*5, corresponding to 109 mm. of mercury. The 

 percentage of carbon dioxide in the alveolar air while, as already 

 remarked (p. 239), very constant in a given individual, varies in 

 different men from 4-6 to 6'2 (mean 5-5) per cent, of the dry alveolar 

 air. In women and in children of both sexes it is less than in men. 

 From this we conclude that in men the partial pressure of carbon 

 dioxide in the alveoli may be at least one-eighteenth of an atmo- 

 sphere, or 42 mm. of mercury (Fitzgerald and Haldane). 



If we take the average oxygen tension in the alveolar air as 

 100 mm., it is clear that the slope of pressure is very decidedly from the 

 alveoli to the venous blood coming to the lungs, the average oxygen 

 tension in the observations with the pulmonary catheter being only 

 37 -7 mm. It must be clearly pointed out, however, that in the lungs 

 the air is in relation with arterialized as well as with venous blood; 

 and if the partial pressure of oxygen in the alveoli, while exceed- 

 ing that in the venous blood, is inferior to that in the arterial blood, 

 the only conclusion which could be drawn would be that some of 

 the oxygen might pass into the blood by diffusion, but that the 

 whole of it could not do so. For as soon as the oxygen tension in 

 the blood, as it became better and better oxygenated in its circuit 

 through the lungs, reached the level of the alveolar partial pressure, 

 diffusion would, of course, come to an end. According to the 

 majority of observers, however, the diffusion hypothesis surmounts 

 this test also, since the oxygen tension in the alveoli is invariably 

 at least as great as that in the arterial blood. Bohr, however, 

 found that in the majority of his observations on dogs the oxygen 

 tension was distinctly greater in the arterial blood than in the pul- 

 monary air. Even if we accept Bohr's results, and they have been 

 severely criticized, the conclusion that the alveolar oxygen tension 

 far exceeds that in the blood of the right heart is in no way affected, 

 and this establishes the possibility of a large absorption of oxygen 

 by the venous blood in the lungs through diffusion alone. It must 

 be carefully remembered that even if it be admitted that diffusion 

 can account for the absorption of the whole of the oxygen, this is 

 not of itself a proof that it is by diffusion that the thing is actually 

 done; it is only a reason for refusing to call in the aid of a more 



