246 RESPIRATION 



rate at which the gas is absorbed, while, for short experiments, 

 the difference in CO pressure between the alveolar air and the 

 blood is constant. In this way we can tell how much CO is ab- 

 sorbed per minute with a given pressure difference; and from 

 this, allowing for the greater solubility and slightly lower dif- 

 fusibility of oxygen, we can calculate the rate at which oxygen 

 diffuses in with the same pressure difference. 



Bohr's original calculations (based on rather rough experi- 

 ments made by myself for another purpose) were not very ac- 

 curate ; but the matter was reinvestigated by A. and M. Krogh, 30 

 and still more recently by M. Krogh. 31 A. and M. Krogh found 

 that for adults about 25 cc. of oxygen will diffuse inwards per 

 minute for every I mm. of difference in oxygen pressure during 

 rest, and about 35 cc. during work. The estimate of M. Krogh is 

 considerably higher ; but I do not think that the method which she 

 used was at all reliable, for the following reasons. The method 

 consisted in taking in a deep breath of air containing a small 

 percentage of CO. Part of this breath was then breathed out, and 

 a sample of the alveolar air taken. The rest of the breath was 

 held for a measured interval of time, after which a second sample 

 of alveolar air was taken, and the percentages of CO in the two 

 samples very accurately determined. From the fall in the per- 

 centage of CO between the two samples the rate of absorption of 

 the CO was then calculated. 



If the difference between the percentages of CO in the two 

 samples represented absorption of CO, the method would be a 

 correct one. Actually, however, it is quite impossible, as I have 

 convinced myself by repeated experiments with various gas mix- 

 tures, to secure an even distribution of a gas through the lung air 

 by taking in a single deep breath. The first alveolar sample con- 

 tains an undue proportion of the atrial air containing a higher 

 initial percentage of CO, while the second sample comes ex- 

 clusively from the alveoli of the air-sac system, in which the per- 

 centage of CO was never nearly so high as in the atria. Thus the 

 apparent absorption of CO during the interval of holding the 

 breath is much greater than the actual absorption. The method is 

 thus fallacious; and the same criticism applies to a number of 

 other Copenhagen experiments with regard to alveolar air, the 

 dead space in breathing, etc. 



Taking, however, the earlier estimate of A. and M. Krogh, it 



30 A. and M. Krogh, Skand. Arch. f. Physiol., XXIII, p. 236, 1910. 



31 M. Krogh, Journ. of Physwl., XLIX, p. 271, 1915. 



