TRANSPORT OF THE RESPIRATORY GASES 101 



Oxyhaemoglobin is a stronger acid than reduced haemoglobin 

 and consequently the equilibrium of the reactions is displaced 

 with the result that CO 2 is liberated from the carbonic acid 

 (fig. 28). It is clear, therefore, that the transport of oxygen and 

 of CO 2 are complementary and provide an extremely efficient 

 mechanism, not only for the uptake of oxygen and liberation of 

 CO 2 in the respiratory organ but also for the liberation of oxygen 

 in proportion to its requirement by the tissues. Furthermore, the 

 presence of bicarbonate within the blood provides the so-called 

 alkali reserve of the body. This is because bicarbonate forms a 

 defence for the body against the production of excess acid. It is 

 not the only alkali reserve in the body, for the proteins can 

 neutralise considerable amounts of acid. Considered as a 

 physico-chemical system the ratio of bicarbonate ions to free 

 CO 2 is only about 1/20, which is a poor buffer-ratio. Neverthe- 

 less, because the nervous regulation is tuned to maintain a 

 constant pH and CO 2 content of the blood this physiological or 

 heterogeneous buffer action transforms the carbonic acid- 

 bicarbonate system from a rather poor buffer into a very good 

 one. The lungs, of course, provide a path by which a great 

 excess of acid in the form of CO2 is removed from the body. In 

 this system, which is vital for the maintenance of the acid-base 

 balance of the body, the corpuscles are most important in the 

 formation of bicarbonate and the plasma is best for its storage. 



The function of blood pigments in raising the colloid osmotic 

 pressure of the blood is absent from vertebrates because the 

 pigment is contained within the corpuscles. As we have seen, this 

 adaptation reduces the viscosity of the blood and yet preserves its 

 high oxygen-carrying capacity. 



