148 RESPIRATION 



of the mixed arterial blood with oxygen is or may be distinctly 

 lower than corresponds to the oxygen pressure of the alveolar air. 



Poulton and I found that when a small quantity of air about 

 6 liters was rebreathed continuously up to the verge of loss of 

 consciousness, the CO 2 being completely absorbed by soda lime, 

 the inspired air contained only 4.8 per cent of oxygen, and the 

 alveolar air 3.7 per cent. There was very great hyperpnoea; for 

 the preformed CO 2 had not had time to escape in the manner 

 already referred to in Chapter VI. The respiratory quotient of 

 the alveolar air was as high as 2.8. The experiment was then 

 repeated with a large volume of air, and under such conditions 

 that the oxygen percentage only fell very slowly. The lowest per- 

 centage of oxygen that could now be reached in the inspired air 

 without great confusion of mind was about 9.4, with about 4.6 

 per cent (or 33 mm.) in the alveolar air. There was no noticeable 

 hyperpnoea, and the respiratory quotient was normal. The al- 

 veolar CO 2 percentage was only reduced from the normal of 5.7 

 per cent to 4.6, indicating that the alveolar ventilation was only 

 increased by about a fourth. 



From these experiments we may conclude that air containing 

 less than 9.5 per cent of oxygen would ordinarily cause disable- 

 ment within half an hour. At a barometric pressure of 368 mm., or 

 a little less than half an atmosphere, corresponding to about 

 20,500 feet above sea level, there would be a corresponding drop 

 in the alveolar oxygen pressure; but judging from my own ob- 

 servations the physiological effects are very distinctly less severe. 

 This is probably due to the fact that in rarefied air the diffusion of 

 oxygen within the lung alveoli is much more free than at atmos- 

 pheric pressure. 9 As a rule no very serious symptoms are ex- 

 perienced at the time till the barometric pressure has fallen to 

 about 350 mm. (corresponding to 21,500 feet) ; but in this respect 

 different individuals vary considerably. It must also be borne in 

 mind that nervous symptoms of anoxaemia begin to appear at 

 altitudes not nearly so great. At 320 mm. (about 24,000 feet) 

 most persons, including myself, are soon very seriously affected 

 in the manner described in Chapter VI, unless they are acclima- 

 tized. 



Another cause of imperfect oxygenation of the arterial blood 

 is that there may not be sufficient time for the required quantity 

 of oxygen to pass into the blood through the alveolar epithelium. 

 This cause of anoxaemia came into prominence in connection with 



* Haldane, Kellas, and Kennaway, Journ. of Physiol., LIU, p. 195, 1915. 



