40 RESPIRATION 



that during rest with normal breathing it was necessary to expire 

 about 800 cc. of air before a reliable alveolar sample could be 

 obtained at the end of inspiration. If the breathing was deep and 

 slow much more air had to be expired. At the end of a normal 

 expiration, however, the air issuing from the mouth is practically 

 alveolar in composition. 



The conclusion reached by Priestley, Douglas, and myself that 

 increased production of CO 2 , and consequent rise in the alveolar 

 CO 2 percentage, determines increased breathing during muscular 

 work was afterwards questioned by Krogh and Lindhard, 22 on 

 the ground that our determinations of the alveolar CO 2 percentage 

 were fallacious, and that the real alveolar CO 2 percentage during 

 muscular work is not only lower than we found, but also con- 

 siderably lower than during rest. Their argument is mainly 

 based on the assumptions, which have already been shown to be 

 wrong, that the "effective dead space" is not largely increased 

 during deep breathing, and that reliable samples of alveolar air 

 can be obtained at the end of a deep inspiration, without more 

 than a very shallow expiration to clear the extra dead space. This 

 part of their argument falls to the ground. They point out, how- 

 ever, what is a real source of slight error namely that a delay of 

 fully half a second occurs during the taking of an alveolar sample, 

 and that during this interval the alveolar CO 2 percentage must 

 rise appreciably. It was shown above that the difference in CO 2 

 percentage between samples of alveolar air taken at the beginning 

 and end of expiration during work corresponding to an increase 

 of 4.3 times in the CO 2 production was about 0.6 per cent. As an 

 expiration took nearly 2 seconds, there would be a rise of 0.15 per 

 cent in half a second, corresponding to the delay in taking the 

 alveolar sample. During rest, according to a similar calculation, 

 there would be a rise of 0.05 per cent. The net error in comparing 

 rest with work would thus be only about o. I per cent, a difference 

 too small to affect the conclusions materially. Owing to their 

 defective methods of estimating and directly determining the 

 alveolar CO 2 percentage at the beginning of expiration Krogh 

 and Lindhard enormously overestimated the error due to a delay 

 of half a second in obtaining a sample. The fact remains, however, 

 that when the work was pushed in the case of Douglas, and even 

 without pushing the work in my own case, the rise in alveolar CO 2 

 percentage was less than corresponded to the increase in breathing. 

 This significant fact will be discussed later. 



22 Krogh and Lindhard, Journ. of Physiol., XLVIII, p. 30, 1913. 



