96 



RESPIRATORY MECHANISMS 



would be likely to occur in the living organism. It is a sig- 

 nificant fact that the effect is very pronounced at low oxygen 

 tensions and practically disappears at full saturation. A brief 

 characterization of the affinity of a certain kind of blood for 

 oxygen which is, however, sufficient for many biological 

 purposes (Krogh and Leitch, 1919), can be given by the ten- 

 sions of "loading" (t t ) and "unloading" (t u ). The loading 

 tension is defined as the tension which will allow the blood to 



Fig. 54. Oxygen dissociation curves of dog's blood at different C0 2 tensions. 



(Krogh.) 



become 95% saturated with oxygen, a figure which comes 

 very close to the normal saturation of arterial blood, while the 

 "unloading" tension corresponds to half saturation. This 

 does not mean of course that the blood becomes necessarily 

 half saturated in the passage through the tissues, but is only a 

 convenient way of expressing the relative facility with which 

 oxygen is given off from the haemoglobin. A high t u increases 

 availability of the oxygen. In Fig. 53 the t u of human haemo- 

 globin at 15°C is about 0.2 mm, while at 38° it is 8 mm. In 

 the actual human blood the t u is about 30 mm. 



Generally the loading and unloading tensions are not 

 independent variables, but a high t t is associated with a 

 relatively high t u and vice versa. This means a conflict of 

 interest in the economy of the organism where it is desirable 

 that the // is as low as possible to insure saturation in the 



