THE BLOOD 103 



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 t 



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, 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 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 epithehum 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 

 probabihty the same explanation — epithehal 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 greater than in the atmosphere. 

 In the swim-bladder of fishes (which is analogous to the lungs of 

 mammals) the oxygen is certainly far in excess of an^iihing that can be 

 explained by mere diffusion. The stoi'age of oxygen, moreover, ceases 

 when the vagus nerves which supply the swim-bladder are divided. 



Some continental observers have stated that certain noxious 

 substances are ordinarily contained in expired air which are much 

 more poisonous than carbonic acid, but researches in this country 

 have entirely failed to substantiate this. If precautions be taken by 

 absolute cleanUness to prevent admixture of the air with exhalations 

 from skin, teeth, and clothes, the expued air only contains one noxious 

 substance, and that is carbonic acid. 



Tissue-Respiration. — Before the processes of respiration were fully 

 understood the lungs were looked upon as the seat of combustion ; they 

 were regarded as the stove for the rest of the body where effete 

 material was brought by the venous blood to be burnt up. When it 

 was shown that the venous blood going to the lungs already contained 



