HEMOGLOBIN EQUILIBRIA 261 



hemoglobin to oxygen to change as combination proceeds. Thus, 

 the investigation of the affinity of hemoglobin for oxygen under all 

 possible combinations of structural and environmental conditions is 

 an impossible task. Most of our knowledge has been built up from 

 relatively restricted data and although great progress has been made 

 by assuming that the influence of a particular factor under one set 

 of conditions may be extrapolated to other conditions, we shall see 

 that the picture built up on this basis is incomplete, and the experi- 

 mental investigation of a number of points is urgently required. 



Different methods have been used for the determination of the 

 oxygen dissociation curve of hemoglobin; Haldane, Barcroft, van 

 Slyke, and Roughton have been chiefly concerned with the develop- 

 ment of the refined gasometric methods now available; while Hiifner, 

 Hartridge, Heilmeyer, Hill, Millikan, and Drabkin have developed 

 spectroscopic and spectrophotometric methods. Some estimations, 

 for example, that of small amounts of carbon monoxide in the presence 

 of a large excess of oxyhemoglobin, can only be carried out accurately 

 by gasometric methods. These methods are somewhat tedious and 

 require a larger amount of manual skill than do other methods such 

 as spectrocolorimetry or visual or photoelectric spectrophotometry. 

 The most recent technique applied to the determination of the equi- 

 librium has made use of the difference in magnetic susceptibility 

 between the compounds (500). 



The oxygen saturation (y) of hemoglobin, is defined by the equation: 



y = [Hb02]/[Hb + HbOo] 



Accurate measurement of the equilibrium is difficult in the regions 

 of low and high oxygen saturation and the choice between one theo- 

 retical expression for the dissociation curve and the other must lie, 

 therefore, on the accuracy with which they describe the dissociation 

 curve between the limits y = 0.1 and y = 0.95. Outside this range, 

 adequate experimental data are not available. This is unfortunate, 

 since most of the equations put forward are able to describe the 

 dissociation curve reasonably well over the middle range of values- 

 and the choice between one equation and another rests more on the 

 general assumption from which they have been constructed, than 

 on the accuracy with which they describe the curve. 



Some of the theories developed apply equally to other equilibria in 

 which hemoglobin is involved, such as the dissociation of carboxy- 

 hemoglobin or the oxidation-reduction system hemoglobin-hemi- 



