RESPIRATION. [CH. xxiv. 



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 figures ; 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 figures (from Fredericq) give the tension of 

 carbonic dioxide in percentages of an atmosphere : 



Tissues 5 to 9 ) ^ 



Venous blood . . . . 3 - 8 to 5 '4 V in dog. y 



Alveolar air .... 2 '8 I I 



External air .... 0^03 



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 

 figures given : 



Arterial blood ..... 



Venous blood 



Alveolar air 



Expired air 



It will be seen from these figures 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 osmosis. Osmosis, however, should cease when 

 the tension of the gas in the blood and 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 ultimately less (2*8) than in the alveolar air. 



The whole question is beset with great difficulties and contra- 

 dictions. Analyses by different observers have given very 

 different results, but if such figures 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. 

 Recent 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. In the swim- 

 bladder of fishes (which is analogous to the lungs of mammals) the 



