82 RESPIRATION 



resulted, and fitted the experimentally determined curve very 

 closely. 32 



1.65 (985) 



(I-S) (I+2S) 



where p = pressure in percentages of one atmosphere, and 

 5 = fractional saturation of the haemoglobin with oxy- 

 gen. 



Thus if 5 be 50 per cent = = y 2 , p will be 4.0, as we actu- 

 ally found to be the case. To express the result in millimeters of 

 mercury pressure, p must of course be multiplied by 7.6, and would 

 thus become, in the above example, 30.4. 



As explained above, the simple dissociation curves for oxy- 

 haemoglobin or CO-haemoglobin in normal human blood 33 are, 

 so far as our present knowledge goes, the same, when allowance 

 is made for the differing affinities of the two gases for haemo- 

 globin. The above equation may therefore be generalized in the 

 form 



1.65 (985) 



pa = 



(i S) (i+25) 



taking a as representing the affinity of the gas for haemoglobin 

 as compared with the affinity represented in the curve on Figure 

 20, giving half-saturation with a gas pressure of 4.0 per cent of an 

 atmosphere. Thus for the fourth curve on Figure 21 (dissociation 

 curve of CO-haemoglobin in the blood of Douglas, in presence of 

 42 mm. CO 2 pressure), at half-saturation pa = 4.0. Hence as p 

 was .017, a was 235, or the affinity of the haemoglobin for the CO 

 (determined without taking into account the solubilities of CO and 

 O 2 ) was 235 times its affinity for oxygen in the standard curve of 

 Figure 20. This is a convenient and easily intelligible method of 

 putting the results. 



82 In working out this equation it was assumed that (as found by Barcroft and 

 Roberts for dogs' haemoglobin) a dialysed solution of the haemoglobin of Douglas 

 and myself becomes half -saturated with oxygen at 38 C and a pressure of 1.6 per 

 cent of an atmosphere of oxygen, and that in human blood saturated with oxygen 

 2/3 of the oxyhaemoglobin is aggregated, and in completely reduced blood 8/9 of 

 the reduced haemoglobin. The curve of the dialysed solution would give the 



i-S 



equation p = 



i. 60 



38 For abnormal human blood the curves are probably different, as will be 

 pointed out in Chapter VIII. 



