RESPIRATION 165 



pressure the more oxygen combines with haemoglobin for the 

 same pressure of oxygen. 



We therefore see that the oxygen and carbon dioxide 

 pressures aid each other in connection with the absorption 

 and liberation of oxygen by haemoglobin. 



The action of carbon dioxide is believed to be due to its 

 acidity. Thus all solutions containing alkaline salts show a 

 curve which is shifted in the opposite direction from those with 

 increasing concentrations of carbon dioxide. There is one 

 pair of salts which require special mention. These are 

 potassium and sodium chlorides. Barcroft found that 

 haemoglobin dissolved in equimolecular solutions of these 

 salts gave different curves. The potassium chloride solution 

 behaved as if it were more alkaline than the sodium chloride 

 solution. 



The explanation of these results seems to be that the 

 haemoglobin is an amphoteric substance with more marked 

 acidic than basic properties. The addition of oxygen to the 

 haemoglobin makes it a stronger acid, just as sulphuric acid 

 is stronger than sulphurous and nitric than nitrous. In 

 support of this hypothesis the experiments of Christiansen, 

 Douglas and Haldane can be quoted, who find that increase 

 in oxygen tension of blood causes an increase in carbon 

 dioxide tension. The increase in oxygen tension makes the 

 haemoglobin into a stronger acid. More alkali combines with 

 haemoglobin, and there is less alkali left to combine with 

 carbon dioxide. Therefore carbon dioxide is set free and 

 the carbon dioxide tension rises.* 



The effect of increase in acidity in favouring the reduction 

 of oxyhaemoglobin can be explained on the Law of Le Chatelier 

 (see pp. 59, 112), because the increase in acidity will prevent the 

 change which causes an increase in acidity, i.e. the combina- 

 tion of oxygen with haemoglobin to form oxyhaemoglobin. 



Barcroft and Hill have suggested that the effect of salts on 

 haemoglobin is due to an effect on the degree of aggregation 

 of the haemoglobin. At the isoelectric point haemoglobin, 

 like other amphoteric colloids, has a greater tendency to run 

 together to form larger particles containing a greater number 

 of molecules than when the solution is more acid or alkaline. 

 The normal haemoglobin is alkaline, hence the addition of 

 acid causes the system to approach the isoelectric point of 

 haemoglobin. By the running together of haemoglobin the 



* J. Christiansen, C. G. Douglas and J. S. Haldane, Journ. Physiol., 

 1914, vol. 48, p. 244. 



