258 A MANUAL OF PHYSIOLOGY . 



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

 made to breathe air containing carbon dioxide in various pro- 

 portions, 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 conclusion 

 that carbon dioxide does not pass through the walls of the 

 alveoli solely by diffusion. And although Bohr's experiments 

 have been severely criticized, it does not seem improbable in 

 itself that the physical process of diffusion, which undoubtedly 

 plays a great part, is aided by some other process, which may 

 provisionally be termed secretion. It is possible, too, that when 

 the conditions 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. 

 But the method by which it was obtained, although correct in 

 principle, has not escaped criticism as to its details (Osborne). 



Additional evidence in favour of the view that there is, 

 besides diffusion, an element of selective secretion in the ex- 

 change of gases through the pulmonary membrane is afforded 

 by a study of the gases of the swim-bladder in fishes. These 



* The subject of the experiment breathes air containing a definitely 

 known very small percentage of carbon monoxide until the haemoglobin 

 has united with as much of that gas as it will take up for the given con- 

 centration of it in the air. Then the percentage amount to which the 

 haemoglobin has become saturated with carbon monoxide is ^determined 

 in a sample of blood taken, say, from the finger. Now, the final saturation 

 with carbon monoxide of a haemoglobin solution brought into contact with 

 a gaseous mixture containing carbon monoxide and oxygen, depends on 

 the relative tensions of the two gases in the liquid. But the tension of 

 carbon monoxide in the blood leaving the lungs will (after absorption has 

 ceased) be the same as that in the inspired air. Knowing this tension and 

 the degree of saturation of the haemoglobin with carbon monoxide, the 

 oxygen tension in the blood leaving the lungs i.e., in the arterial blood 

 is known. 



