178 HEMOGLOBIN 



irreconcilable with any view of the essence of the reaction between 

 oxygen and haemoglobin other than that of its being a chemical 

 combination : 



Thermodynamical reasoning shows that if the pressure of oxygen required to keep 

 an haemoglobin solution at a constant degree of saturation is equal to p^ when the 

 temperature is T^, and the pressure required to maintain the same degree of saturation 

 is P2 when the temperature is T2, the heat of reaction (Q) per gram mol. of oxygen 

 is given by the formula below: 



■^ 1 ~ -^ 2 Pi 



This equation is independent of the mechanism of the reaction — it applies to the 

 heats of adsorption of gases on charcoal as well as to chemical reactions. 



There is, however, an important difference between the heats of adsorption and 

 the heat of reaction of haemoglobin. 



In adsorption Q is large at low saturations with gas and it diminishes as the pressure 

 is increased. In the case of haemoglobin, increase of temperature alters the scale of 



the curve but not its shape. That is to say, the ratio — at a low saturation is the 



Pz 



same as the ratio — measured for a higher saturation, and according to the thermo- 



P2 

 dynamic formula, the heat of reaction Q must be the same at all degrees of oxygena- 

 tion. This conclusion applies to blood and to certain haemoglobin solutions, but it does 

 not necessarily apply to solutions in which a large change in jpH takes place on 

 oxygenation. 



To pass to another point. Granting that the only difference between 

 the curves in Fig. 57 is the horizontal scale on which they are drawn 

 we may proceed to consider some other of their properties. The 

 pressures at which the haemoglobin is 50 per cent, saturated are for 

 the temperatures given as follows : 



Temperature (° C.) 14 26 32 38 



Pressure for half -saturation (mm.) 2 8 11 22 



These pressures are related to the equihbrium constants of the 

 reaction. In Henderson's (7) view they would be proportional to the 

 equihbrium constants K, for the various temperatures, in Hill's they 

 would be proportional to the reciprocals of \^K in his equation. 



In either case we should be able to find the heat evolved when 

 one molecule of oxygen unites with haemoglobin (according to 

 Henderson's view with one molecule, according to Hill's view with 



- molecule of haemoglobin). 

 n 



If these pressures be considered proportional to the equihbrium 

 constants of the reactions of oxygen mth haemoglobia (i.e. the velocity 

 of association divided by the velocity of dissociation — the equihbrium 



