RESPIRATION 147 



to a saturation of about 80 per cent of the haemoglobin with oxy- 

 gen, and that any further diminution will cause a rapid fall in the 

 saturation. The air produces at the time no noticeable discomfort, 

 and the breathing is not sensibly affected, although the lips are 

 slightly bluish. The natural conclusion is that a diminution of 

 about 15 per cent in the saturation of the haemoglobin, or a dimi- 

 nution to half in the arterial oxygen pressure, is of no physio- 

 logical importance, even though the lips are rather dull in color. 

 This wholly mistaken idea is, however, rudely shaken by the 

 effects of remaining for a sufficient time in the atmosphere : for 

 the observer will be almost certainly prostrated by an attack of 

 mountain sickness which he is never likely to forget afterwards. 



If, now, in order to escape mountain sickness, the pressure of 

 oxygen in the inspired air is only diminished by one-seventh (cor- 

 responding to a height of 4,500 feet; or an oxygen percentage of 

 1 7 at ordinary atmospheric pressure) , there will be no appreciable 

 blueness, and the corresponding saturation on the oxyhaemoglobin 

 dissociation curve will be only 3.5 per cent below that for normal 

 alveolar air. Nevertheless there will, if sufficient time is given, be 

 quite appreciable physiological responses, which .will be discussed 

 in succeeding chapters. The truth is that in the long run the body 

 responds in a fairly delicate manner to quite small diminutions 

 in the oxygen pressure of the inspired air. 



Let us now look at the matter in the light of the new knowledge 

 as to the somewhat imperfect manner in which air is distributed 

 in the alveoli. In the course of our investigation on military 

 neurasthenia, we placed several of the patients in a steel chamber 

 and observed the effects of diminished pressure. A very slight dim- 

 inution, corresponding to only about 5,000 feet, was sufficient to 

 produce in them urgent respiratory and other symptoms, although 

 they were doing no work. Even in normal persons the dissociation 

 curve of oxyhaemoglobin and composition of the mixed alveolar 

 air are, as was shown above, no certain guides to the percentage 

 saturation of the haemoglobin, or oxygen pressure in the mixed 

 arterial blood. As a matter of fact the blueness of the lips seen in 

 persons freshly exposed to very low atmospheric pressure seems 

 to be often much greater than would correspond to the oxygen 

 pressure in their alveolar air when due allowance is made for the 

 Bohr effect of lowered alveolar CO2 pressure. We may thus be 

 quite sure that at diminished atmospheric pressure the saturation 

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

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



