252 SCIENCE PROGRESS. 



theory ; for if there is not time for gaseous equilibrium to 

 establish itself between the blood and the air durino- rest, it 

 is difficult to see how the blood could be completely aerated 

 during muscular work, when the amount of oxygen absorbed 

 by the lung in a given time is many times greater, although 

 the alveolar oxygen-tension is about the same. Yet we 

 know from the experiments of Geppert and Zuntz that during 

 muscular work the blood drawn from an artery is if anything 

 more completely aerated than during rest. 



Against the probability of active absorption by the lung 

 it might be urged that the production of a higher oxygen 

 tension in the blood than in the alveolar air would be use- 

 less, since even at the latter oxygen tension the haemoglobin 

 would, practically speaking, be completely saturated with 

 oxygen. This reasoning depends on the assumption that 

 the only function of the oxygen absorbed by the lungs is to 

 saturate the haemoglobin of the venous blood. There is 

 now, however, strong evidence to shovv that the oxygen has 

 other essential functions to perform, and that at any rate 

 the maintenance of a high oxygen tension is essential to 

 health. We found that any great fall in the normal oxygen 

 tension is accompanied by symptoms which cannot be attri- 

 buted simply to deficient saturation of the haemoglobin with 

 oxygen. Possibly the high oxygen tension is in some way 

 connected with the very considerable formation of carbonic 

 acid which, according to the recent experiments of Bohr and 

 Henriques, occurs within the lungs. 1 



On a review of all the investigations relating to the 

 causes of respiratory exchange between the air in the lungs 

 and the blood it is clear that the balance of evidence is at 

 present strongly in favour of the view that the lung epithe- 

 lium participates actively in the process. Until, however, 

 the causes of the apparent discrepancies in the results ob- 

 tained by different observers have been satisfactorily cleared 

 up, we shall do well to regard our conclusions on this, as on 

 so many other questions in physiology, as only provisional. 



J. S. Haldane. 



1 Archives de Physiologic, 1897, p. 590. 



