74 RESPIRATION 



constant. It thus appears that the effect of substituting CO for O 2 , 

 or of varying the partial pressure of CO 2 , is only to alter a simple 

 constant in the equation to the curve. In other words it is only 

 the affinity of haemoglobin for the gas saturating it which alters. 

 With respect to the oxyhaemoglobin curve the same conclusion 

 was reached by Barcroft and Poulton, 26 who found that variations 

 in the partial pressure of CO 2 had, within wide limits, the same 

 effects on the dissociation curve of oxyhaemoglobin, as on that of 

 CO-haemoglobin. In the case of Bancroft's blood it requires a 

 little over twice as high a partial pressure of oxygen to produce 

 half-saturation of the haemoglobin in presence of 40 mm. pres- 

 sure of CO 2 as when CO 2 is absent; just as in the blood of Douglas 

 it takes a little over twice as high a partial pressure of CO. Bar- 

 croft and Means 27 have, however, also shown that in the case of a 

 salt-free or nearly salt-free solution of haemoglobin the effect of 

 CO 2 is not merely to alter the affinity of oxygen for haemoglobin, 

 but also to alter the mathematical form of the curve, just as salts 

 do. Hence it is only in the case of whole blood that the affinity 

 alone is altered ; and probably we should find that it is only within 

 definite limits of variations in the hydrogen ion concentration of 

 whole blood that the mathematical form of the dissociation curve 

 is sensibly unaltered. 



When blood or haemoglobin solution is exposed to a mixture 

 of CO and air the haemoglobin becomes partly saturated with CO 

 and for the rest with O 2 . I found many years ago that with a dilute 

 solution of blood the curve representing the percentage saturation 

 of the haemoglobin with CO when increasing percentages of CO 

 are added to the air in the saturating vessel is a rectangular hyper- 

 bola. 28 Figure 22 shows curves obtained by Douglas and myself 

 with undiluted blood at body temperature from two persons and 

 two mice. 29 



It will be seen that in each case the curve is a rectangular 

 hyperbola, corresponding to the simple reversible reaction HbO 2 

 + CO^HbCO + O 2 . Thus for my own blood the proportions of 

 HbCO to HbO 2 are I : I with .07 per cent of CO, 2 : 1 with 2 x .07 

 per cent of CO, 3 : 1 with 3 x .07 per cent of CO, etc. For each kind 



*" Barcroft and Poulton, Journ. of Physwl., XLVI, Proc. Physwl. Soc., p. iv, 

 1913- 



87 Barcroft and Means, Journ. of Physwl., XLVII, Proc. Physwl. Soc., p. 

 xxvii, 1914. 



"Haldane, Journ. of Physiol., Vol. XVIII, p. 449, 1895. 



28 Journ. of Physiol., Vol. XLIV, p. 278, 1912. 



