194 HEMOGLOBIN 



not matter and there is nothing to make it matter. In that case 

 it comes to be as it is because the particular globin with which the 

 hsematin is united is not of significance from the point of view of 

 temperature. 



So much then for the fact that haemoglobin transports oxygen. 

 It remains to be said that the equivalent of about 17,000 grams 

 transports only 32 grams of oxygen and therefore great solubiUty is 

 required. Here again we find that the solubiUty is conferred on the 

 iron-containing hsematin — a very insoluble substance — by the pro- 

 tein, and that among proteins globin is far the most efficient, so that 

 globin performs the double function of rendering the oxygen labile 

 and the molecule soluble. 



We now come to the third of the fundamental properties with 

 which this discussion was started, namely, that the pressures at 

 which the oxygen is acquired and given up should be such that the 

 oxidation of the haemoglobin is as nearly as possible complete in the 

 limg and its reduction easy in the capillaries of the tissues. Krogh 

 has introduced a nomenclature to express the point numerically. 

 He calls the partial pressm-e of oxygen to which the haemoglobin is 

 exposed in the lung or gill "the tension of loading," and that at which 

 the haemoglobin becomes 50 per cent, reduced "the tension of un- 

 loading." With this nomenclature at our service let us review the 

 position. When considering the temperature coefiicients we assumed 

 the haemoglobin to be in the fundamental condition in which such 

 things must be considered if a start is to be made in the investigation 

 of their properties, namely, in a dilute solution under some sort of 

 standard circumstances. The evidence on the subject is not beyond 

 reproach, but at present it seems to poiut to the dissociation curve 

 being a hyperbola under such fiuidamental circumstances. This, if 

 true, is of great interest, firstly, because a ciurve of that shape is 

 extremely ill-suited to the needs of the intensive form of respiration 

 which alone makes the warm-blooded animal as we know it 

 possible, and secondly, because the hyperbolic curve forms a simple 

 starting-point from which we may hope to derive the more com- 

 phcated curves that are actually foimd in the body. Not only may 

 we hope to derive these curves but we may hope also to discern the 

 process of evolution which nature has fixed upon in order to obtain 

 a curve suitable to her piupose. 



Firstly, then, as regards the most suitable combinations of tensions 

 for loading and imloading. It is clear that the tension of loading 



