RESPIRATION 161 



per cent, and the affinity of haemoglobin for CO (in my own case 

 at least) is about 300 times its affinity for oxygen. It follows that, 

 if we assume for the moment that the oxygen pressure of the blood 

 is that of the normal alveolar air, the blood will gradually become 



half-saturated with CO if air containing = .047 per cent of 



300 



CO is breathed continuously for a sufficient time. If the per- 

 centage is .0235 per cent, the final saturation will only be one 

 third ; and if the percentage is .012 the saturation will be a fourth ; 

 and so on. If pure air were again breathed the CO would be ex- 

 pelled from the body through the unbalanced action of the al- 

 veolar oxygen pressure in expelling CO from its combination. The 

 rates of absorption and of elimination of the CO can also be 

 calculated on the same principles from the mean percentage of 

 CO in the alveolar air, allowing for the fact that as the haemo- 

 globin approaches the balancing saturation the rate of absorption 

 will gradually fall off; and similarly the rate of elimination will 

 gradually fall off as the blood loses CO. As will be shown in 

 Chapter IX, however, this theoretical course of events is pro- 

 foundly modified by active secretion of oxygen inwards by the 

 lung epithelium. 



It is evident also that in air abnormally poor in oxygen a given 

 percentage of CO will become more poisonous, and in air ab- 

 normally rich in oxygen less poisonous. This I verified experi- 

 mentally on animals. It remained to ascertain in man what effects 

 corresponded to a given saturation of the haemoglobin ; and this I 

 ascertained by experiments on myself, 21 using for the purpose the 

 carmine titration method referred to in Chapter IV, and fully 

 described in its latest form in the Appendix. 



I found in these experiments that no particular effect was ob- 

 served until the haemoglobin was about 20 per cent saturated. At 

 about this saturation an extra exertion, such as running upstairs, 

 produced a very slight feeling of dizziness and some extra palpita- 

 tion and hyperpnoea. At about 30 per cent saturation very slight 

 symptoms, such as slight increase of pulse rate, deeper breathing, 

 and slight palpitations, became observable during rest, and run- 

 ning upstairs was followed in about half a minute by dizziness, 

 dimness of vision, and abnormally increased breathing and pulse 

 rate. At 40 per cent saturation these symptoms were more marked, 

 and exertions had to be made with caution for fear of fainting. 

 At 50 per cent saturation there was no real discomfort during 



21 Haldane, Journ. of PhyswL, XVIII, p. 430, 1895. 



