192 Chapter XII 



represents the actual measurement of the tensions of oxygen and 

 carbonic acid which he obtained, and which are not inconsistent 

 with the facts as stated above. So far, therefore, as our knowledge 

 of the difference of pressure necessary to maintain an adequate 

 current of oxygen through the lung is concerned, we may assume 

 that the oxygen pressure in the blood cannot exceed about 1 mm. 

 less than that in the alveolar air for every 100 c.c. of oxygen 

 absorbed by the organism, but we are at present somewhat in the 

 dark as to how much it is less than that amount. 



The invasion coefficient applies only to the one portion of the 

 physical process of the passage of a gas through the lung, namely the 

 entrance of the oxygen into the moist surface. Bohr (3) made some 

 experiments shortly before the close of his life upon the physical 

 constants involved in the whole process a subject which had pre- 

 viously been investigated by Hiifner' 4 ', and Zuntz and Loewy (5) . 

 What pressure head is necessary to force the amount of oxygen 

 which can be taken up by the human lung through the epithelium 

 in the time? The method of experiment is as follows: Carbon 

 monoxide is assumed to pass through the lung by diffusion, it is 

 then taken up by the haemoglobin. The amount that is taken up in 

 a given time can be measured. Within certain limits the pressure of 

 CO produced in the blood may be neglected. The amount which 

 diffuses through will then depend simply upon the external pressure 

 of CO. Now the pressure of CO in the alveolar air and the volume 

 of CO which passes through the epithelium in a given time are 

 both measurable and may be related directly to one another. This 

 relation is called the diffusion coefficient for the lung. When it is 

 once determined for CO it is assumed to have been determined for 

 oxygen from the known relation of the diffusion constants of the two 

 gases in physical experiments outside the body. 



According to Bohr the diffusion coefficient for the lungs Avas of 

 such an order as to allow of the actual quantity of oxygen used at rest 

 to pass through the epithelium at the usual alveolar oxygen pressure, 

 but if his determination of the diffusion coefficient were correct it 

 would be impossible for the amount of gas which traverses the epi- 

 thelium during exercise to pass by diffusion, as also the quantity of 

 oxygen which goes through at rest, when the person is exposed to 

 low pressures of oxygen. 



To sum up the situation as it was three or four years ago. The 

 diffusion theory rested upon aerotonometer experiments, which for 



