RESPIRATION 243 



training should have little difficulty from anoxaemia. It must be 

 noted, however, that even a small degree of the neurasthenia with 

 shallow breathing described in Chapter III renders an airman 

 totally incapable of going to any considerable height without an 

 oxygen apparatus. 



Our acclimatization experiment indicated that with complete 

 acclimatization, including adjustment of the blood reaction to 

 the increased breathing, and increase in the haemoglobin per- 

 centage, a man could probably, if the mere physical difficulties 

 were not too great, reach the summit of Mount Everest without 

 breathing anything else than ordinary air, though he would 

 quite certainly die at this altitude if he were not acclimatized. 



It was pointed out in Chapters III and VII that, on account of 

 the imperfect distribution of air in the lungs, the average alveolar 

 oxygen pressure is, even during rest under normal healthy condi- 

 tions, no certain guide to the oxygen pressure of the mixed 

 arterial blood. During heavy work this must be so to an increased 

 degree, since, although the expansion of the lungs is much better, 

 the rate at which oxygen is absorbed is enormously greater. 

 Meakins and Davies^^ have recently made exact determinations 

 of the percentage saturation with oxygen of the haemoglobin in 

 the arterial blood of a number of healthy persons, and found it to 

 vary from 94 to 96 per cent in different persons, the variation 

 depending probably on the differences in the oxyhaemoglobin 

 curves which Barcroft discovered (Chapter IV). In my own case 

 the saturation was 94.3 per cent. This is not much lower than 96 

 per cent, the saturation which would be expected if my arterial 

 blood were fully saturated to the oxygen pressure of the mixed 

 alveolar air. If, however, we look at the dissociation curve of 

 the oxyhaemoglobin of human blood, we see that 94.3 per cent 

 saturation corresponds to an oxygen pressure of only 1 1.2 per 

 cent of an atmosphere, as compared with 13.2 per cent in the 

 alveolar air. Thus the oxygen pressure in the mixed arterial blood 

 is very distinctly less than in the alveolar air ; and this is the sort 

 of result which the aerotonometer gives, as already explained. 



On the other hand the carbon monoxide method gives, during 

 rest under normal conditions, exactly the same oxygen pressure 

 in the arterial blood as in the alveolar air. This difference in the 

 results by the two methods used to be rather a puzzle, and was 

 explained by me as probably due, either to a process of rapid but 



*® Meakins and Davies, Journ. of Pathol, and, Bacter., XXIII, p. 453, 1920. 



