RESPIRATION 165 



with the haemoglobin below 20 per cent of its normal value, and 

 the lips of extremest pallor, the patient is perfectly conscious, 

 though hardly capable of any muscular exertion. 



The key to this seeming paradox is furnished by the discovery^^ 

 that the oxyhaemoglobin left in the arterial blood when it is 

 partially saturated with CO has its dissociation curve altered in 

 such a way that the haemoglobin holds on more tightly to the oxy- 

 gen. The oxygen still present as oxyhaemoglobin is therefore less 

 easily available, so that the oxygen pressure in the tissues must 

 fall lower in order to get off the combined oxygen. With a given 

 amount of available oxygen in the blood the physiological anox- 

 aemia is thus increased. Figure 52, from a paper by J. B. S. Hal- 

 dane,^^ shows the alterations in the dissociation curves of the 

 oxyhaemoglobin with varying percentage saturations of the blood 

 with CO. It will be seen, for instance, that with 50 per cent satu- 

 ration of the blood with CO the oxygen pressure must fall to less 

 than half the usual value, and with 75 per cent saturation to less 

 than a third, in order to dissociate half the oxygen present in the 

 arterial blood as oxyhaemoglobin. No wonder, therefore, that the 

 symptoms of CO poisoning are much more severe than those of a 

 corresponding simple deficiency of haemoglobin in the blood. It 

 will be seen also that the shape of the dissociation curve is com- 

 pletely altered. The characteristic double bend (which, as already 

 seen, is of such vital physiological importance) in the oxyhaemo- 

 globin curve tends to disappear altogether, so that an enormous 

 fall in oxygen pressure is needed to make the bulk of the oxygen 

 in the oxyhaemoglobin dissociate. 



In the investigations which Lorrain Smith and I made on the 

 effects of continuously breathing a definite percentage of CO all 

 the experiments were made on ourselves, and in a series which 

 was more or less continuous from day to day. From the results of 

 these experiments we estimated that it required about .05 per cent 

 of CO in the air to produce the 30 per cent saturation of the blood 

 which was necessary for any very noticeable symptoms of CO 

 poisoning. In isolated experiments made later, however, we found 

 the CO much more poisonous, so that it only required about .02 

 per cent to produce the required saturation. In the original ex- 

 periments we had become "acclimatized" without knowing it. The 

 great significance of this "acclimatization" will be discussed in 

 succeeding chapters. 



" Douglas, J. S. Haldane, and J. B. S. Haladne, Journ. of Physiol., XLIV, 



293, 1912. 



"J. B. S. Haldane, Journ. of Physiol., XLV, Proc. Physiol. Sac, p. xxii, 19 12. 



