198 



Chester XIII 



have meant shirking a difficulty that would surely have asserted 

 itself sooner or later. 



To recall the statement made on p. 191 



10 20 30 W 50 



spjredair 



220 3ff 



10 



20 30 40 



FIG. 98. Oxygen and C0 9 pressures _ in the air at the bifurcation of the bronchi, 



in the blood. Ordinate pressure of oxygen expressed in percentages of an 



atmosphere. Abscissa = time. (Krogh.) 



The margin referred to above is of course (p p'}, its magnitude 

 is not less than and it varies directly with the quantity of 



gas going through the epithelium. In the case to which I have 

 referred M is taken in man as 400 c.c. of oxygen per minute. It is 

 known, however, that 2000 c.c. of oxygen per minute is not an 

 excessive amount in cases of hard exercise in which case (pp'} 

 would have to be increased 5-fold, that is, to at least 125 mm., in 

 other words p' would disappear altogether, seeing that even with 

 forced respiration 125 mm. probably exceeds the oxygen pressure in 

 the alveolar air. 



