J 



112 RESPIRATION 



increased breathing, by washing out CO2, checks this increase 

 very quickly, so that the net result for the time is only a small 

 increase. Where the anoxaemia is only slight this net increase will 

 be practically inappreciable, and this, as will be shown in Chap- 

 ter VIII, is due, not to the fact that there is no appreciable anox- 

 aemia, but to the masking of the natural response to anoxaemia 

 by the opposite response to the washing out of COg. After a suffi- 

 cient interval of time the former response, as we shall see, becomes 

 unmasked by the compensation of the latter response, so that in 

 the long run there is a very definite response of the breathing to 

 even a very small fall in the oxygen pressure of the inspired air. 



When diminution in the oxygen pressure of the inspired air is 

 accompanied by a corresponding increase in the pressure of carbon 

 dioxide, it is evident that within wide limits the pressure of oxy- 

 gen in the alveolar air will remain almost normal, since the in- 

 creased breathing due to the extra carbon dioxide will so raise 

 the alveolar oxygen pressure as to compensate approximately for 

 the oxygen deficiency in the inspired air. There will thus be no 

 appreciable anoxaemia, and consequently the oxygen deficiency 

 in the inspired air will produce no effect at all, although a similar 

 deficiency in the absence of the excess of CO2 would produce a 

 marked effect. For instance, by adding CO2 to the inspired air we 

 can easily compensate within wide limits for the deficient oxygen 

 pressure which affects airmen at high altitudes. This is not be- 

 cause, as Mosso^ imagined, the effects of high altitude are due 

 primarily to excessive loss of CO2 ("acapnia"), but because the 

 oxygen pressure, as well as that of CO2, is kept approximately 

 constant by the increased breathing due to the CO2. When, how- 

 ever, the conditions are such that the extra breathing due to ex- 

 cess of CO2 does not prevent the alveolar oxygen pressure from 

 falling very low, the stimulus of anoxaemia is added to that of 

 CO2, and an enormously greater effect is produced on the breath- 

 \ ing than by the CO2 stimulus alone. This extra effect, as was 

 recently shown by Meakins, Priestley, and myself^ is due to in- 

 crease in the frequency of the breathing; and increased frequency, 

 provided the depth of breathing is sufficient, is, for a reason which 

 will appear in the next chapter, particularly effective in prevent- 

 ing anoxaemia. 



A further complication in the effects of anoxaemia and forced 

 breathing on the respiratory center and the body as a whole is 



* Mosso, Life of Man on the High Alps, Chapter XXII, London, 1898. 

 *Haldane, Meakins, and Priestley, Journ. of Physiol., LII, p. 420, 1919. 



