RESPIRATION 



239 



only during work that the experimental results showed secretion; 

 but as a matter of fact the increase found in the arterial oxygen 

 pressure above the alveolar oxygen pressure would be of very 

 little service in charging the blood further with oxygen ; and this 

 brings us back to the original question. 



In the first place it must be pointed out that the experiments 

 which Douglas and I made on oxygen secretion during muscular 

 work were carefully arranged in such a way as to demonstrate the 

 existence of secretion if any secretion occurred during muscular 

 work. We knew already that the stimulus to oxygen secretion 

 came from anoxaemia of the tissues. We knew, also, that the 

 only probable function of oxygen secretion was, not to raise the 

 arterial oxygen above that of the alveolar air, but to prevent a 

 serious fall, such as otherwise might take place during work suffi- 

 cient to increase very greatly the oxygen requirements of the 

 body. But we had no index of what the fall might be in the absence 

 of secretion. We therefore made the experiments in such a way 

 that tiring work, such as would presumably furnish the stimulant 

 to oxygen secretion, was done with one arm only. The oxygen 

 requirements of the body were in this way only increased to a 

 very moderate extent, so that oxygen secretion would have every 

 chance to raise the arterial oxygen pressure above the alveolar 

 oxygen pressure, just as in CO poisoning or at high altitudes dur- 

 ing rest. It was also much easier to make the experiments accu- 

 rately when the oxygen intake was not greatly increased. 



Since our original experiments, and those on Pike's Peak, were 

 carried out, a good deal of both direct and indirect evidence has 

 accumulated in confirmation of our conclusions, and must now be 

 referred to. In Chapter VII the very clear physiological evidence 

 was summarized, showing that there is, in persons who are not in 

 good physical training, considerable anoxaemia during hard mus- 

 cular exertion. This is not merely local anoxaemia in the muscles 

 with the associated formation of lactic acid described in Chapter 

 VIII : for if the work is not too hard the respiratory symptoms 

 indicating anoxaemia are still present during the work, but there 

 is no trace of a subsequent fall in the resting alveolar CO2 pres- 

 sure. This fall is the physiological indication of lactic acid, and 

 runs parallel, as already mentioned, with the presence of much 

 lactic acid in the blood and urine. As Douglas and his pupils have 

 found,^^ there is, as a matter of fact, practically no increase in 



"Campbell, Douglas, and Hobson, Phil. Trans. Roy. Soc, (B), Vol. 210, p. i, 

 1920. 



