2 The Causes of Absorption of Oxygen by the Lungs in Man. 



the principle of the experiment being otherwise the same as in the 

 experiments on animals. 



The following results have been obtained : — 



(1) During rest under normal conditions, and provided that the blood is 

 not more than about 25 per cent, saturated with carbon monoxide, the 

 partial pressure of oxygen in the arterial blood is practically identical with 

 that in the alveolar air. This result accords completely with the theory 

 that under these conditions the absorption of oxygen is by diffusion alone. 



(2) When the percentage of oxygen in the inspired air is lowered 

 sufficiently (or the saturation of the blood with carbon monoxide is increased 

 sufficiently) to cause appreciable symptoms of want of oxygen, the partial 

 pressure of oxygen in the arterial blood becomes very considerably higher 

 than in the alveolar air. Active secretion of oxygen inwards is therefore 

 occurring, as was formerly concluded by Haldane and Lorrain Smith 

 from experiments on mice. We find, however, that Haldane and Lorrain 

 Smith's results require a considerable and at present somewhat uncertain 

 correction. 



(3) During muscular work also, unless the work was of a comparatively 

 gentle kind, a similar result was obtained, and muscular work with the 

 inspired air poor in oxygen seemed to produce a specially striking effect. 



Taken together, the results indicate that the lung epithelium is excited 

 directly or indirectly to active secretion of oxygen inwards by products of 

 metabolism proceeding from the muscles and other tissues when their oxygen 

 supply is insufficient to meet ordinary requirements. That such insufficiency 

 actually occurs during muscular work, and when air with a low partial 

 pressure of oxygen is breathed, has already been shown.* These results are 

 of special interest in connection with the phenomena of adaptation to very 

 high altitudes, and throw a new light on the physiology of mountain climbing 

 and balloon ascents, and of ordinary muscular work. 



* Geppert and Zuntz, ' Pfliiger's Archiv,' 1888, vol. 42, p. 189 ; Boycott and Haldane, 

 ' Journ. of Physiol.,' 1908, vol. 37, p. 355 ; Ogier Ward, ibid., vol. 37, p. 378 ; Haldane 

 and Poulton, ibid., vol. 37, p. 390 ; Douglas and Haldane, ibid., 1909, vol. 38, p. 420 ; 

 Byffel, ibid., vol. 38, p. 29 ; Boycott and Chisolm, ' Biochemical Journ.,' 1910. 



