320 UNITY AND DIVERSITY IN BIOCHEMISTRY 



the result of the action of a muhitude of factors such as the p^^Q and tem- 

 perature of the arterial blood, the affinity of the haemoglobin for oxygen, 

 the intensity of the Bohr effect, etc. 



Corpuscles containing haemoglobin are found in the blood of all Verte- 

 brates, and since these animals have a well-defined respiratory cycle they are 

 a useful starting-point for the study of the role of haemoglobin in the 

 transport of oxygen by the blood. 



The important part played by haemoglobin in oxygen transport is clearly 

 shown by the values obtained for different bloods of the "transported 

 oxygen", i.e. the difference in total oxygen between the venous and arterial 

 blood less the difference in dissolved oxygen for these bloods. 



The part played in oxygen transport by the sigmoid character of the 

 dissociation curve is evident. With the partial pressures of oxygen occurring 

 in arterial and venous blood, a hyperbolic curve would only allow the 

 transport of a much smaller amount of oxygen. When the reduction of the 

 blood is accompanied by acidification the Bohr effect also comes in, since 

 the curve is displaced to the right, so that, at the same pQ^ there is a lower 

 degree of oxygenation of haemoglobin. 



Does the dissociation curve, as it exists under arterial and under venous 

 conditions, show any characteristics parallel to the physiological or ecolo- 

 gical character of the animal? 



If, regardless of the particular oxygen carrier they contain, we examine 

 the few animal species whose blood respiratory cycle is known, and if we 

 note the values of the arterial p^^ and the degree of saturation of the arterial 

 blood, we shall find that from one case to another there exists, even for 

 animals living in the same environment, very different gradients of oxygen 

 pressures, contrary to what is found with CO2 which is always, or nearly 

 always, in the arterial blood, in equilibrium, with the external environment. 

 Yet, whatever the p^^ resulting from the particular character of the res- 

 piratory system, the arterial blood is always found to be 90 to 98% saturated. 

 In other words, whatever the difference between the external Pq^ and the 

 />o^, the point on the dissociation curve corresponding to the arterial Pq^ 

 is always found to lie along the top right-hand portion of the curve. 



Hence the shape and position of the dissociation curve are adapted to 

 the respiratory needs of the organism. The slightest decrease in the />o, 

 would immediately bring about a loss of oxygen from the haemoglobin. 

 The fact that one is not justified in considering, even in aquatic animals, 

 that the arterial /)q, is equal to the external ^q_ has been verified by the 

 observations of H. M. Fox (see Table XVII). 



Despite the fact that we know little about the respiratory cycle of fish, 

 it is possible to establish a relation between the more or less vertical shape 

 of their dissociation curve and the concentration of oxygen in the external 

 environment. First proposed in 1919 by Krogh and Leitch, this relation has 



