FUNCTION OF CELL CHLORIDES 337 



This means that a considerable proportion of the respiratory 

 carbon-dioxide (quite 90 per cent, of the total respiratory carbon- 

 dioxide carried by the plasma) is transported in combination with 

 bases of the plasma, which previous to the loading up with carbon 

 dioxide w^re combined with hydrochloric acid. While the respira- 

 tory carbon-dioxide is " on board " this hydrochloric acid is carried 

 in the cells in combination with base liberated from haemoglobin. 

 In determining the actual contribution of the haemoglobin to the 

 transport of respiratory carbon-dioxide, it is therefore necessary to 

 take into account not only the excess of carbon-dioxide in the cells of 

 venous blood over that in arterial blood, but also the excess of cell 

 chlorides. When this is done it is found that, as stated above, the 

 haemoglobin provides for the transport of at least 80 per cent, of 

 the respiratory carbon-dioxide. 



When the carbon-dioxide is excreted in the lung, the processes 

 described above occur, of course, in the reverse order, although, 

 owing to the fact that under all physiological conditions plasma 

 contains a higher concentration of both chlorides and carbonates 

 than the cells, this involves the passage of chlorides and car- 

 bonates from a lower to a higher concentration. 



This simple explanation of the mechanism of the transport of 

 carbon-dioxide needs, therefore, further exploration, since it would 

 seem to entail first the displacement of the strong acid hydrochloric 

 by the weak acid carbonic in the plasma, and then later the replace- 

 ment in the cells of the relatively stronger acids hydrochloric and 

 carbonic by the very weak acid haemoglobin (which even in its 

 oxygenated form is still a very weak acid). It is well knozvn that 

 a weak acid will replace a strong one in combination if the strong one 

 is removed as it is formed, and, as indicated above, it is on this basis 

 that the removal of carbon-dioxide from the blood, in the lung, is 

 explained. But it is necessary to consider why the displacement 

 of chlorides should take place, since there is no variation in chloride 

 tension in lungs and tissues. 



It has been stated that carbonic acid displaces the stronger 

 hydrochloric acid from combination as the chlorides of the plasma, 

 because the hydrochloric acid so liberated diffuses through the cell 

 membrane and is thus removed from the sphere of action, and that 

 therefore the reaction NaCl + HoCOg r^ NaHCOg + HCl pro- 

 ceeds from left to right. It must, however, be remembered that 

 the cell membrane is freely permeable also to carbon-dioxide, and 

 that, in fact, at least 40 per cent, of respiratory carbon-dioxide 

 passes into the cell and there displaces base from combination with 

 haemoglobin. Since, moreover, enough base can be liberated from 

 combination with haemoglobin to combine with the whole of the 



