242 A MANUAL OF PHYSIOLOGY 



would represent the partial pressure of the carbon dioxide 

 in the blood rather than in the alveolar air under normal 

 conditions. For gaseous equilibrium is soon established 

 between blood and air separated only by a thin membrane 

 like the alveolar wall. 



In Bohr's experiments, in some of which the animals 

 were made to breathe air containing carbon dioxide in 

 various proportions, the tension of that gas in the air of 

 the lungs varied from 5*8 to 34*6 mm. of mercury, while in 

 arterial blood, taken at the same time, it usually ranged 

 from 10 to 38 mm., and was often less than in the alveolar air. 



If we accept these results, we seem shut up to the con- 

 clusion that carbon dioxide does not pass through the walls 

 of the alveoli solely by diffusion. And although Bohr's 

 experiments have been severely criticised, it does not seem 

 improbable in itself that the physical process of diffusion is 

 aided by some other process, which may provisionally be 

 termed secretion. It is possible, too, that when the con- 

 ditions are especially unfavourable to diffusion when, for 

 instance, the partial pressure of carbon dioxide is artificially 

 increased in the alveoli the cells which line them are 

 stimulated to increased activity. 



As to the oxygen, we are in the same position. Its partial 

 pressure does not appear to be always higher, even under 

 normal conditions, in the alveoli than in the arterial blood 

 as it leaves the lungs. Indeed, Bohr found that, in the 

 majority of his observations on dogs, the oxygen tension 

 was distinctly greater in the blood than in the pulmonary 

 air. And Haldane and Smith, using a new method, have 

 obtained a value for the oxygen tension in human blood 

 (26*2 per cent., equal to 200 mm. of mercury) that even 

 exceeds the partial pressure of oxygen in the external air, 

 and is about twice as great as that of the air of the alveoli. 

 This remarkable result cannot be reconciled with any purely 

 physical explanation of the absorption of oxygen. The relative 

 excess of the oxygen-tension in arterial blood over the alveolar 

 oxygen-tension increases as the proportion of oxygen in the 

 alveolar air is diminished. 



Additional evidence in favour of the view that there is, 

 besides diffusion, an element of selective secretion in the 



