The acquisition of oxygen by the blood in the lung 201 



value. This value is nearly seven times that given by Bohr. If 

 therefore the calculation of p p' for the human lung was repeated 

 with the new coefficient, it would seem to be not 25 mm. but about 

 3 4 mm. As a very rough calculation of the invasion coefficient 

 shows that it demands a difference of pressure on the two sides of 

 the lung surface of 1 mm. for every hundred c.c. absorbed by the 

 lung per min., it may seem to the reader that I have gone at undue 

 length into the invasion coefficient and that a matter which has been 

 proved by Krogh to be so trifling does not demand the extensive 

 discussion which I have given to it. This is so, perhaps, under the 

 ordinary conditions of life, but let him remember that at high altitudes 

 millimetres of oxygen pressure become precious. They are at best 

 all too few. If they are whittled away in passing through the lung 

 and if this process is magnified by the exact proportion of the amount 

 of work which is expended in climbing, then the reader will agree 

 with me that we do well to keep account of them one by one. 



When the oxygen has entered the surface of the lung a further 

 pressure gradient must exist in order to drive it through the cells of 

 the epithelium. About this we have few reliable theoretical data. 



In rabbits we have seen that the whole pressure head, as measured 

 experimentally by Krogh, amounts to 15 20 mm. (Fig. 98). How far 

 this reflects the condition of affairs in man we do not know. Rabbits 

 like all small animals use up a proportionately large amount of oxygen ; 

 other things being equal, man would use up less, but we have no 

 certain knowledge what the relative surfaces of the lung in man 

 may be as compared with the rabbit. 



If we imagine a man situated at an altitude of 15,000 feet, his 

 alveolar oxygen pressure will be about 50 mm. of mercury. If now 

 we suppose him to be at rest and deduct 10 mm. from this for the 

 pressure head necessary to maintain the diffusion, the pressure of 

 oxygen in his blood will be 40 mm., and if further we suppose that 

 he does an increased amount of work, requiring twice the amount 

 of oxygen, and turn this 10 mm. into twenty we leave only a 

 possible 30 mm. as the oxygen pressure in the arterial blood. This 

 seems an impossibly low figure. However, as we have said, these 

 values which we have been using are speculative and 40 mm. oxygen 

 pressure would place the organism in a very different position from 

 30 mm. But the calculation illustrates the desirability of at once 

 doing experiments upon man himself and finding out under extreme 

 circumstances which, though abnormal, can exist, what the oxygen 



