130 ESSENTIALS OK CHEMICAL PHYSIOLOGY 



to form disodium phosphate (Na 2 HPO 4 ). In this way, as soon as 

 the amount of free carbonic acid diminishes, as in the lungs, the 

 amount of carbonic acid in combination also decreases ; whereas in 

 the tissues, where the tension of the gas is highest, a large amount is 

 taken up into the blood, where it forms sodium bicarbonate. 



The tension of the carbonic acid in the tissues is high, but one 

 cannot give exact ngures ; we can measure the tension of the gas in 

 certain secretions ; in the urine it is 9, in the bile 7 per cent. The 

 tension in the cells themselves must be higher still. 



The following ngures (from Fredericq) give the tension of carbonic 

 dioxide in percentages of an atmosphere : 



Tissues 5 to 9 | 



Venous blood .... 3*8 to 5 '4 ^in dog ^ 



Alveolar air .... 2*8 j | 



External air . . . . 0'04 



The arrow indicates the direction in which the gas passes : namely, 

 in the direction of pressure from the tissues to the atmosphere. 



In some other experiments, also on dogs, the following are the 

 ngures given : 



Arterial blood 2-8 



Venous blood . . '. . . " . 5*4 & 



Alveolar air ...... 3 - 56 j 



Expired air . . . . . . 2*8 



It will be seen from these ngures that the tension of carbonic acid 

 in the venous blood (5'4) is higher than in the alveolar air (3'56) ; its 

 passage into the alveolar air is therefore intelligible by the laws of 

 diffusion. Diffusion, however, should cease when the tension of the 

 gas in the blood and in the alveolar air are equal. But the transference 

 goes beyond the establishment of such an equilibrium, for the tension 

 of the gas in the blood continues to sink until it is, when the blood is 

 arterial, ultimately less (2*8) than in the alveolar air. 



The whole question is beset with great difficulties and contradic- 

 tions. Analyses by different observers have given very different 

 results, but if such ngures as those just quoted are ultimately found 

 to be correct, we can only explain this apparent reversal of a law 

 of nature by supposing with Bohr, that the alveolar epithelium 

 possesses the power of excreting carbonic acid, just as the cells of 

 secreting glands are able to select certain materials from the blood 

 and reject others. Eecent work by Bohr and Haldane has also 

 shown that in all probability the same explanation epithelial 

 activity must be called in to account for the absorption of oxygen. 

 Haldane, in fact, states that the tension of oxygen in the blood is 



