RESPIRATION 



247 



oxaemia during considerable exertion is therefore exactly what 

 might be anticipated on the diffusion theory, even without any 

 allowance for the effects of uneven distribution of air and blood 

 among different alveoli. When allowance is also made for this 

 factor, the presence of anoxaemia during even very moderate 

 exertion at ordinary atmospheric pressure in persons not physi- 

 cally fit is just what might be expected; and at high altitudes the 

 anoxaemia would be so serious as to make any considerable ex- 

 ertion impossible but for active secretion. 



All the facts, therefore, and not merely our direct measurements, 

 go towards showing that oxygen secretion is a most important 

 physiological factor, not merely under exceptional circumstances, 

 but during ordinary life at sea level. It is probably also an im- 

 portant factor under pathological conditions, though on this sub- 

 ject our knowledge is still almost a blank, owing to lack of 

 observations. The only relevant observations are those of Lorrain 

 Smith. ^^ His experiments, when due allowance is made for the 

 errors already referred to in our calculations, showed that either 

 a rise of body temperature or a severe infection paralyzed the 

 power of oxygen secretion in response to CO poisoning. When 

 lung inflammation was produced by exposing the animals to a 

 high pressure of oxygen (see Chapter XII) the arterial oxygen 

 pressure fell to values which, when corrected, are much below 

 that of the alveolar air. In this case it is evident that not only 

 active secretion, but also diffusion of oxygen inwards, was. inter- 

 fered with. The animals were incapable of muscular exertion and 

 thus showed symptoms similar to those of phosgene poisoning, as 

 described in Chapter VII. 



A significant determination has quite recently been published 

 by Harrop^^ of the percentage saturation of human arterial blood 

 with oxygen, first during rest, and then just after exhausting 

 work. The results were 95.6 per cent during rest, and 85.5 per 

 cent just after the exertion. The deficiency found in the blood just 

 after exertion is far greater than could be accounted for by ex- 

 perimental errors. 



As already mentioned, the aerotonometer experiments of Krogh 

 indicated that the arterial CO2 pressure is the same as that of the 

 alveolar air. The manner in which the respiratory center responds 

 to the slightest increase or diminution in the alveolar CO2 pres- 

 sure, and the quantitative correspondence between rise in alveolar 



''Lorrain Smith, Journ. of Physiol., XXII, p. 307, 1898. 

 " Harrop, Journ. of Exper. Med., XXX, p. 246, 19 19. 



