RESPIRATION 83 



The corresponding equation worked out by Hill is 



y_ 



ioo zz 



where x = oxygen pressure in mm. of mercury, 



y =i percentage saturation of the haemoglobin, 

 K = a constant varying for different curves. 



For the blood of Douglas (which was the first to be investi- 

 gated completely by Barcroft, and which was also investigated by 

 ourselves) the value of K was . 000196. 34 



Hill's equation gives curves almost identical with ours, and as 

 he had kindly communicated it to us by letter we should certainly 

 have adopted it had we seen how the theory on which it is based 

 could be brought into definite relation with the particular rec- 

 tangular hyperbola given by dialysed haemoglobin, or reconciled 

 with the fact that the dissociation curve of CO -haemoglobin in 

 presence of a constant oxygen pressure is a rectangular hyperbola. 

 Hill soon afterwards offered a possible explanation as regards the 

 latter point. 35 It seems to me that this explanation is improbable, 

 but so also, it must be confessed, are certain assumptions connected 

 with the deduction of our own equation. At present the data are 

 lacking for a decision as to whether either theory is correct, al- 

 though both equations are for all practical purposes satisfactory. 

 I cannot see, however, how to escape the conclusion that there is 

 more aggregation among the unsaturated than among the satu- 

 rated molecules of haemoglobin. It is evident that far more data 

 are needed to enable us to understand the dissociation of oxy- 

 haemoglobin in blood. 



With the help of the chemical facts described in the present 

 chapter we might proceed at once to the discussion of a number of 

 physiological and pathological problems; but such a discussion 

 would be incomplete and misleading in the absence of the facts 

 relating to the carriage of CO 2 by the blood, and this subject will 

 therefore be considered in the next chapter. 



34 The value of K as calculated from our own results (Fig. 20) is, for the blood 

 of both Douglas and myself, outside the normal limits given by Barcroft and rep- 

 resented graphically in Figure 109, page 226, of his book The Respiratory Function 

 of the Blood, The cause of this discrepancy is not yet clear. 



"A. V. Hill, Bio-Chemical Journal, VII, p. 471, 1913. 



