260 RESPIRATION 



the scale stand the results of Haldane and Smith, who found in mart 

 an oxygen tension in the arterial blood of over 200 mm. of mercury 

 equal to more than 26 per cent, of an atmosphere. This exceeds 

 the partial pressure of oxygen in the external air, and is about twice 

 as great as that of the air of the alveoli. In the bird they found 

 an oxygen tension of between 300 and 400 mm., equal to 45 per 

 cent, of an atmosphere. These results, however, differ so vastly 

 from those of all other observers that for the present it is best to 

 leave them out of account. The method by which they were ob- 

 tained, although perhaps correct enough in principle, seems to be 

 exposed to several sources of error in practice, and has not escaped 

 criticism as to its details (Osborne, etc.). We are left, then, with a 

 series of values for the oxygen tension of arterial blood which lie 

 always below the partial pressure of the gas in atmospheric air, and 

 usually do not much exceed or fall much below 100 mm. of mercury, 

 corresponding to about 13 per cent, of oxygen. 



The average tension of carbon dioxide in the venous blood passing 

 through the lungs, as determined by the pulmonary catheter, in 

 man was 45 mm., corresponding to 6 per cent, of an atmosphere. 

 This agrees fairly well with most of the observations made with the 

 aerotonometer. The lower results (of Wolffberg and of Nussbaum) 

 with the lung catheter are probably due to the fact that in the dogs 

 used, which breathed through tracheal cannulae, the catheter caused 

 greater interference with the respiration than in man and induced 

 dyspnoea, with the consequent washing out of carbon dioxide (p. 246) . 



The chief interest of this discussion of the blood-gas tensions lies 

 in their fundamental importance in the problem of the gaseous 

 exchange in the lungs, on the one hand, and between the blood and 

 tissues on the other. We are now in a position to consider the former. 



Calculations made on the basis of such anatomical and physical 

 data as are available (total surface of the lungs, thickness of the 

 membrane which separates the air of the alveoli and the blood in 

 the capillaries, velocity of diffusion of oxygen and carbon dioxide), 

 indicate that even with differences of oxygen tension between the 

 blood and the alveolar air, which would lie within the limits of 

 error of our present methods of measurement, enough oxygen 

 could diffuse across the pulmonary membrane to cover the whole 

 normal intake. The speed of diffusion of carbon dioxide across 

 such a membrane being much greater than that of oxygen, still 

 smaller differences of tension would suffice to permit the whole 

 normal output of that gas to be eliminated by diffusion. Accord- 

 ingly, the problem in its present phase reduces itself to this, whether, 

 as a matter of fact, the slope of the oxygen pressure is always from 

 the alveolar air to the blood passing through the lungs, and the 

 slope of the carbon dioxide pressure always from the blood to the 

 alveolar air ? 



