352 LOADING UP 



quently begins at altitudes of 2,000 to 3,000 metres, particularly if 

 the ascent has been fairly rapid by railway. Airmen usually suffer 

 after ascending 5,000 to 6,000 metres. The partial pressure of 

 oxygen in the blood as determined by the carbon-monoxide method 

 has been found to be 35 mm. Hg above the oxygen pressure of 

 alveolar air. Considerable doubt exists, however, as to the validity 

 of this method. It depends on the careful matching of a carboxy- 

 lated blood with a blood-carmine mixture, and minute quantities 

 of blood are used. 



TABLE LIV 



Effect of Atmospheric Pressure on Alveolar Oxygen 



Tension 



One fact in the data given by those investigators is a little 

 strange although it may have no significance. Notwithstanding 

 that the arterial oxygen tension was always higher than that 

 given for the alveolar air, it was never higher than that of the 

 atmosphere at the time, although occasionally not much below it. 

 Why should the secretory power fail just at this level and not 

 raise the oxygen tension above that of the atmosphere ? Is 

 it possible that the blood had come into equilibrium with an 

 oxygen tension which was not given correctly by the measurement 

 of that of the alveolar air ? Might it not also be possible that 

 the carbon-monoxide method gives different values when the 

 iiaemoglobin content of the blood is increased, as in the case of 

 acclimatisation to high altitudes ? 



TABLE LV 

 Atmospheric Pressure and Number op Erythrocytes 



Hasselbalch shows that the hydrogen ion concentration is 

 increased under those circumstances. 



" This question of secretion by the lungs is instructive from the point 

 of view of ' vitalism.' When first proposed, it was held to apply to the 



