The acquisition of oxygen by the blood in the lung 197 



reduction does take place, corresponding at most to '0007 c.c. of 

 oxygen per c.c. of blood per minute (or about half a per cent, of the 

 oxygen contained in the blood) and at least '0002 c.c. It is in some 

 respects fortunate, in others unfortunate, that Krogh chose the rabbit 

 as the object of this research. The blood of the rabbit was found 

 by Douglas to reduce itself with much greater violence than that of 

 higher mammals. If, therefore, the self-reduction does not matter in 

 the case of the rabbit and it would seem that it must be very slight 

 in the infinitesimal time taken by blood to traverse a tube joining 

 two ends of the carotid artery it clearly does not matter in the 

 higher types. On the other hand it is probable that if Krogh had 

 used the cat he would have obtained figures probably in the next 

 decimal place and therefore placed self-reduction in that animal far 

 out of the field. 



As regards the actual tensions observed and the comparison of 

 them with the alveolar air, Krogh was at the disadvantage under 

 which all workers in this subject who depend upon animal experiments 

 find themselves, ihat of getting an actual sample of alveolar air to 

 analyse. Calculating it from the air of the bifurcation of the bronchi, 

 he found (1) that the C0 2 pressure in the alveolar air and the blood 

 were the same and kept the same whatever changes took place in the 

 former, (2) that the oxygen pressure in the blood was always below 

 that of the alveolar air by a certain amount, 15 20 mm., and that 

 this margin remained even with considerable variations of oxygen 

 pressure in the alveolar air. 



For the moment I wish to focus the attention of the reader upon 

 this margin of 10 20mm., and to recall the fact that according to 

 Bohr the diffusion theory demanded for the human lung an excess of 

 about 25 mm. of pressure in the alveolar air over that present in the 

 film of moisture on the exposed surface of the lung cells. As well as 

 this a further difference of pressure was required to drive the oxygen 

 through the cells. Owing to the extreme solubility and diffusibility of 

 carbonic acid no measurable margin is demanded between the carbonic 

 acid pressure in the lung and in the alveolar air. Krogh's result then 

 agrees with Bohr's expansion of the diffusion theory inasmuch as he 

 shows a margin in the case of the oxygen and no margin in the case 

 of the carbonic acid. 



The margin in the case of the rabbit was less than what Bohr had 

 postulated in the case of man, but that might easily be so. The 

 question, however, could not be left at this point because it would 



