82 RESPIRATION 



resulted, and fitted the experimentally determined curve very 

 closely. ^^ 



1.65 (9—85) 

 ^ ^ (i_S)(i+25) 



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 = -^ — = ^, /> 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*^ 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 (9-85) 

 ^"-(1-5) (1+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. CO2 pressure), at half-saturation pa = 4.0. Hence as p 

 was .01 7, a was 235, or the affinity of the haemoglobin for the CO 

 (determined without taking into account the solubilities of CO and 

 O2) 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. 



" 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 =z — tt: 



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

 pointed out in Chapter VIII. 



