THE DISSOCIATION CURVE OF HEMOGLOBIN 93 



found. Let us introduce the element of pressure quantitatively by 

 spacing the cylinders apart from one another at distances which are 

 proportional to the concentrations (or pressure) of oxygen dissolved 

 in the solution, as is done in Fig. 21. 



By joining the points which divide the blue and red portions of 

 the cylinders we obtain a curve which relates the percentage of 

 oxyhsemoglobin to the concentration of oxygen dissolved in the 

 fluid at aU concentrations of oxygen between and -0029 cc, and 

 consequently to all pressures of oxygen between and 100 mm. This 

 curve representing the equilibrium between oxygen and haemoglobin 

 is called the dissociation curve of oxyhsemoglobin. It is shown in 

 Fig. 22. 



Let us now turn from the observed properties of haemoglobin to 

 the other side of the question, namely the requirements of the law of 

 mass action. This law has been stated quantitatively by Guldberg 

 and Waage in the following terms: "The velocity of chemical change 

 is proportional to the product of the active masses of the reacting 

 substances." The chemical change is conceived as taking place in 

 both directions at the same time, that is to say oxyhsemoglol^in is 

 all the time being formed and being broken up, and we therefore 

 have two changes taking place at one and the same time, (1) the 

 formation, (2) the breakdown of oxyhaemoglobin. Since these 

 changes balance one another, the whole system being in equilibrium, 

 the velocities of the two changes are equal. Taking first the formation 

 of oxyhaemoglobin: the reacting substances are reduced haemoglobin 

 and oxygen and the velocity of their reaction is proportional, says 

 the law, to the 'product of their concentrations in the solution. If 

 Cq be the concentration of oxygen and Cr of reduced haemoglobin, 

 then the velocity of the reaction is proportional to, or is equal to, a 

 constant h multiplied by the product of C^ and Cq 



= k{CR X Co). 



As regards the other phase of the reaction — the breakdown of 

 oxyhaemoglobin — there is but one active substance, namely oxy- 

 haemoglobin; let its concentration be C^, then the velocity of the 

 reaction is k' , another constant, multiplied by Cg. Taking the two 

 together, we have 



1c {Cr X Co) = h'Cs. 



Th6 concentration of oxygen as we have already stated is « x =-^^ , 



760 



