78 RESPIRATION 



clear that provided we know the relative affinities of the two gases 

 for the haemoglobin, and the pressure at which one is present, we 

 can tell from an observation of the percentage saturation of the 

 haemoglobin the pressure of the other. Hence we can use haemo- 

 globin solutions for determining small percentages of CO in air. 

 All that is necessary is to introduce a little blood solution into a 

 small bottle of the air, shake till the solution takes up no more CO, 

 and then determine colorimetrically the percentage saturation of 

 the haemoglobin with CO, and calculate the percentage of CO 

 present. 30 Still more important in physiological work is the con- 

 verse determination of the oxygen pressure by observation of the 

 percentage saturation of haemoglobin exposed to a constant pres- 

 sure of CO. By this means, as we shall see later, it is possible to 

 measure the partial pressure of oxygen in the arterial blood within 

 the living body and so decide the question whether active secretion 

 of oxygen inwards occurs in the lungs. 



Douglas and I found that when the combined pressure of O 2 

 and CO are insufficient to saturate the haemoglobin the dissocia- 

 tion curve of CO-haemoglobin in presence of a constant pressure 

 of CO and diminishing pressure of O 2 begins to diverge from the 

 rectangular hyperbola which it would otherwise have followed, 

 and then proceeds to trace out the peculiar hump shown on the 

 lower two curves in Figure 23, and in greater detail in Figure 24. 

 We thus have what seems at first sight a most anomalous fact, 

 namely that although all other facts show that increase in the 

 pressure of oxygen tends to keep out CO more and more from 

 combination with haemoglobin, yet at very low pressure of oxygen 

 and CO the reverse is the case, and increase of oxygen pressure 

 helps the CO to combine with haemoglobin. There can be no doubt 

 that the converse is also the case namely that at low pressures of 

 CO the presence of the CO helps the oxygen to combine with the 

 haemoglobin. This explains a very anomalous fact noticed by 

 Lorrain Smith and myself many years ago 31 namely that the 

 presence of a small percentage of CO helps animals to resist the 

 effect of a very low oxygen pressure, or at any rate does not make 

 them worse. We had expected that a given percentage of CO 

 would become more and more poisonous the more the oxygen 

 pressure was diminished, and this was the case within certain 

 limits ; but we were then quite at a loss to understand why with 

 very low oxygen pressures the CO seemed to do no harm. 



""Haldane, Methods of Air Analysis, p. 119, 19 1 g. 



"Haldane and Lorrain Smith, Journ. of Physiol., XXII, p. 246, 1897- 



