310 



Comparative Animal Physiology 



variable number of iron units combining with oxygen, to the interaction be- 

 tween groups, and in part to the effects on hemoglobin of other components of 

 the blood such as the blood salts. Purified hemoglobin gives a more nearly 

 hyperbolic oxygen dissociation curve than does hemoglobin in blood or in the 

 presence of blood salts. The Oo dissociation curve of muscle hemoglobin is 

 more nearly hyperbolic than that of blood hemoglobin, and it lies to the left 

 of the blood curve; that is, myoglobin has a greater oxygen affinity than blood 

 hemoglobin and can take oxygen from it. Dog myoglobin is only 40 per cent 

 dissociated at 5 mm. of Oo; the blood hemoglobin is 95 per cent dissociated 

 at this tension. '^^ Except at moments of stress, myoglobin remains saturated. 



The oxygen tension at which the pigment is 95 per cent or more saturated 

 is called the loading tension (ti) or tension of saturation (tsat); the oxygen 

 tension at which the pigment is 50 per cent saturated, that is, when Hb equals 

 HbOo, is the unloading tension (tu) or tension of half saturation (ti/o sat)- 



30 40 50 



OXYGEN PRESSURE 



Fig. 67. Oxygen dissociation curves in per cent saturation of hemoglobin as a function 

 of oxygen pressure in mm. Hg in a variety of animals. Data assembled by Redfield."^ 



In Table 55 the ti/o ^at and the tsat of a number of animals are compared 

 under nearly physiological conditions. In Figure 67 are plotted oxygen dis- 

 sociation curves of the blood of some animals (pigeon, crocodile) in which 

 the affinity for oxygen is low, i.e., the tj/o sat is high, and of other animals 

 (^Arenicola and eel) which have a great affmity for oxygen, i.e., the ti/2 sat 

 and tsat are low. In Figure 68 similar curves are plotted, using the actual 

 amount of oxygen combined in volumes per cent instead of per cent saturation 

 as ordinate. From the data of Table 55 and of Figures 67 and 68, it is apparent 

 that hemoglobins differ greatly in their oxygen affmity and in the amount of 

 oxygen they can carry when saturated. Unloading tensions set the upper 

 limit of tissue oxygen tension and the lower limit of environmental oxygen 

 for function of the hemoglobin. 



In the bird bloods which have been studied the affinity for O^ is less than in 

 man, i.e., the ti/2 sat and tsat are higher, but the dissociation curves are 



