362 RESPIRATION 



oxygen, the alveolar air in situ would contain 47 per cent of H 2 O ; 

 probably about 20 per cent of CO 2 ; and 33 per cent of oxygen, 

 with a partial pressure of about 4.3 per cent of an atmosphere or 

 33 mm. of mercury. This pressure of oxygen is only one twenty- 

 third of that in dry oxygen at atmospheric pressure, though the 

 oxygen pressure in the inspired oxygen is only reduced to a little 

 over a seventh. 



It is thus somewhat remarkable that until extremely low baro- 

 metric pressures, such as under 100 mm., were reached, the deaths 

 of the animals from want of oxygen should have coincided so 

 closely with a threshold oxygen pressure in the inspired air. The 

 probable explanation of this has already been referred to in 

 Chapter VI. With fall of barometric pressure the rate of dif- 

 fusion in a gas increases rapidly, since the mean free path of 

 each molecule before it strikes another molecule is increased. As 

 a consequence, the oxygen molecules in the neighborhood of the 

 alveolar epithelium reach it more rapidly, so that when there is 

 scarcity of oxygen the blood can be more readily saturated to the 

 existing mean oxygen pressure in the alveoli, or to whatever 

 higher oxygen pressure can be produced by active secretion. The 

 excessive fall in alveolar oxygen pressure at low barometric pres- 

 sures is thus partially compensated. 



An experiment which Paul Bert describes (p. 749 of his book) 

 would seem to confirm this explanation. A bird was placed in the 

 apparatus (Figure 93) and the pressure reduced to 220 mm., at 

 which the animal had severe symptoms of anoxaemia. The pres- 

 sure was then raised to normal, not with air, but with nitrogen. 

 The animal died almost at once, though the partial pressure of 

 oxygen was 6 per cent, and the alveolar oxygen pressure must have 

 been raised, owing to the greatly diminished proportion of aque- 

 ous vapor in the alveolar air at normal barometric pressure. 



The importance of the CO 2 present in the air was not noticed 

 by Paul Bert. In all his experiments where the oxygen pressure 

 of the inspired air fell to about 3.5 per cent before death there 

 was also a considerable proportion of CO 2 in the inspired air. 

 This CO 2 must have stimulated the respiration greatly, in the 

 manner already explained so fully, thus diminishing the fall in 

 alveolar oxygen pressure. The presence of CO 2 tends to diminish 

 the percentage saturation of the haemoglobin in the arterial blood, 

 owing to the Bohr effect already referred to at length in Chapters 

 IV and VII, but there is the counterbalancing advantage that the 

 haemoglobin holds on less tightly to oxygen in the systemic 



