CH. XXIV.] CARBONIC ACID IN THE BLOOD 381 



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 Mn dog. 



Alveolar air . . . . . . 2*8 J 



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 2'8 ^ 



Venous blood 5 '4 ! 



Alveolar air 3'56 I 



Expired air 2-8* 



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 

 diffusion. Diffusion, 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 contradic- 

 tions. 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 

 oxygen is certainly far in excess of anything that can be explained by 

 mere diffusion. The storage of oxygen, moreover, ceases when the 

 vagus nerves which supply the swim-bladder are divided. 



The Ae'rotonometer. This instrument was invented by Pfl. ger, and of the 

 modifications that have been introduced since then, that of Fredericq is the simplest 

 It merely consists of a long glass tube surrounded by a water-jacket at body 

 temperature. The tube is filled with a known mixture of gases, and the blood from 

 the carotid artery is allowed to slowly trickle down the tube ; the blood then returns 

 to the jugular vein. In order that the experiment may be continued for an hour or 

 more, the animal's blood must be rendered incoagulable ; this may be done by a 

 previous injection of "peptone." This has the disadvantage of lowering the car- 

 bonic acid tension of the blood. Another way of rendering the blood incoagulable 

 is to collect it, defibrinate it, and then return it to the circulation. The last step in 



