422 RESPIRATION 



the diluted blood is distinctly less pink than some of the same solution re- 

 saturated with coal gas ; and on titration the blood which has been simply 

 diluted will be found to be not more than 88 or 89 per cent saturated. 

 The percentage dissociation can be calculated if we know the partial 

 pressure of CO corresponding to various percentage saturations of the 

 haemoglobin at room temperature, and also the coefficient of solubility 

 of CO. 



In the case of human blood, half-saturation occurs at room tempera- 

 ture in presence of air with about .05 per cent of CO. Hence with 50 per 

 cent saturation of a blood solution saturated with air the partial pressure 

 of CO will be .05 per cent of an atmosphere. Now 100 cc. of water (and 

 presumably also of a very dilute blood solution) dissolves about 2.5 cc. 

 of CO from an atmosphere of pure CO at room temperature (i5°C.), so 

 that at a partial pressure of .05 per cent it will dissolve 2.5 x .0005 = 

 .00125 cc. of CO, whereas 100 cc. of 0.5 per cent blood solution can take 

 up in chemical combination .0925 cc. of CO. Hence the proportion of the 

 haemoglobin dissociated is .00125 in .0925, or 1.35 per cent, so that if 

 50 per cent saturation were found by titration to be present we should 

 require to add 1.35 per cent to obtain the true result. By a similar 

 calculation we find that if the blood were found by titration to be 89 

 per cent saturated, we should have to add on 11 per cent in order to 

 obtain the true result, which would thus be 100 per cent. When human 

 blood is fully saturated with coal gas, the result actually found by titra- 

 tion, after dilution of the blood to 0.5 per cent, is 89 per cent, provided 

 the light is not bright. Hence the calculation agrees with the actual result. 

 In the brighter light of the middle of the day the result is, however, 2 or 

 3 per cent lower, even with a north light ; and on going outside so as to 

 increase the light, and avoid the absorption of actinic rays by window 

 glass, the result is still lower. This effect is due, as was pointed out by 

 Haldane and Lorrain Smith, to the action of actinic rays in dissociating 

 CO haemoglobin. The varying effect of light renders the carmine titration 

 with very high saturations of the blood with CO somewhat uncertain. 

 With low saturations, such as we have usually worked with, any error due 

 to this cause is trifling. We have at all times avoided bright light as far 

 as possible, and where it was necessary, as in the case of dissociation 

 curves, to titrate with high saturations of the blood, up to 80 or even 

 85 per cent, we have done the titrations by evening light. As an alterna- 

 tive, we might have used narrower test tubes and a greater concentration 

 of the blood solution, so as to diminish the correction for dissociation ; 

 but it is easier to judge the tints accurately when ordinary test tubes are 

 employed, and comparatively few determinations were needed with very 

 high saturations. 



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