RESPIRATION 363 



capillaries. The excess of CO 2 has, however, another quite dis- 

 tinct effect in counterbalancing the effects of the low alveolar oxy- 

 gen pressure : for the circulation can increase, owing to thensthnu- 

 lus of anoxaemia, without the counteracting effect due to the 

 production of alkalosis through deficiency of CO 2 . In this way the 

 oxygen pressure in the systemic capillaries is kept considerably 

 higher than if there were no excess of CO 2 in the inspired air. 



Other things being equal, the presence in the inspired air of 

 a moderate proportion of CO 2 diminishes the effects of oxygen 

 deficiency, as can easily be shown experimentally. The CO 2 , by 

 increasing the breathing, raises the percentage of oxygen in the 

 alveolar air; and a very small excess in the alveolar CO 2 pressure 

 is sufficient to produce a large effect on the breathing. There is 

 consequently a considerable increase in the alveolar oxygen pres- 

 sure. That, however, the effects of CO 2 in relieving anoxaemia 

 are not simply due to the increased oxygenation of the blood can 

 be shown most strikingly in CO poisoning. A given percentage 

 of CO is less poisonous when administered to an animal breathing 

 human expired air. As this does not raise the alveolar oxygen 

 pressure, the effect cannot be due to increased oxygenation of the 

 arterial blood, and must be put down to increase in the circulation 

 rate, and consequent better supply of oxygen to the tissues. Lor- 

 rain Smith and I found that excess of CO 2 has no effect in stimu- 

 lating oxygen secretion by the lungs. 



Although Paul Bert had in reality proved quite conclusively 

 that the physiological effects of low atmospheric pressures depend 

 on the lowering of the oxygen pressure, the theory was promi- 

 nently brought forward by Mosso twenty years later that these 

 effects are due primarily to excessive loss of CO 2 from the body, 

 or "acapnia." Mosso imagined that as a physical consequence of 

 the low atmospheric pressure more CO 2 than usual is washed out 

 of the blood in the lungs, and that this is the cause of mountain 

 sickness. 1 His physical chemistry was completely at fault. If the 

 volume of air breathed did not alter, the partial pressure of CO 2 

 in the alveolar air would remain the same, and no more CO 2 would 

 be given off at low than at ordinary atmospheric pressure. Actu- 

 ally, however, there is an excessive loss of CO 2 at low atmospheric 

 pressure, and this is due to the increased breathing caused by the 

 anoxaemia. Moreover we can, for the reasons already explained, 

 mitigate the anoxaemia by adding a suitable proportion of CO 2 



1 Mosso, Life of Man on the High Alps (translation), London, 1898. 



