DISSOCIATION OF OXY-HAEMOGLOBIN 



249 



where the amount of oxygen in the blood was determined at 

 various pressures. 



TABLE XXXVIII. 



If the tensions (1) are taken as abscissae and the degree of 

 saturation (4) as ordinates, a curve (Fig. 56, heavy line) is 

 obtained called the dissociation curve of blood. This curve is 

 compound and may be resolved into its component factors : 



(a) Pure Haemoglobin. The dissociation curve for pure haemo- 

 globin will be found in Fig. 56, dotted line. It differs from that 

 of whole blood in that its contour is that of a rectangular hyperbola. 



(b) Solutes. The materials in solution in the blood corpuscles 

 and in the plasma are the cause of the difference between the 

 dissociation curve of pure haemoglobin and haemoglobin in blood. 

 If the pure haemoglobin is dissolved in the actual salts which are 

 present in the red blood corpuscles, the curve of the haemoglobin- 

 salt solution will be found to lie, point for point, on that of whole 

 blood. Examination of such curves will demonstrate the value 

 of these salts in the unloading of oxygen from the corpuscle. 



From the curves it will be seen that at the pressure of oxygen in the 

 lung i.e. 90 mm. Hg. both whole blood and pure haemoglobin solution 

 are saturated to about the same extent with oxygen. (To say that they 

 are 92 per cent, saturated is equivalent to saying that 92 per cent, of the 

 haemoglobin has got its full cargo of oxygen, and 8 per cent, has no oxygen 

 at all. This must not be considered as admitting the correctness of such 

 a supposition. It may be that each molecule of haemoglobin has got 

 92 per cent, of its maximum load.) That is, the presence of solutes does 

 not materially modify the oxygen capacity of the blood at high tensions of 

 oxygen. In the tissues, the tension of oxygen is low, say 15 mm. Hg. 



