RES FIR A TION 625 



the haematin constituent. Htematin itself appears to have acid properties, so 

 that it seems difficult to accept this suggestion. 



On the whole, it is clear that much more work is necessary before we can 

 regard the nature of the association between oxygen and haemoglobin as decided. 

 I have felt it necessary to point out where existing hypotheses fail, though it 

 would have been pleasanter to be able to take them as satisfactory. There seems 

 some risk that the question may be considered, prematurely, to be settled. At the 

 same time, I have no alternative hypothesis to suggest, although I cannot help 

 thinking that the subject would repay more investigation from the adsorption 

 point of view than it has yet received. Not having worked at it myself, I hold r.o 

 brief for one side or the other, and cannot claim any particular value for my 

 remarks, which are merely based on the aspects presented to an onlooker. 



The Action of Carbon Monoxide. This gas has, as it is expressed, a much 

 greater " affinity " for haemoglobin than oxygen has, so that, in a mixture of the 

 two gases, there is a much larger amount of carbon monoxide combined with 

 haemoglobin than corresponds to the relative tension of the two gases. At the 

 same time, there is a definite law regulating the proportion, which has been made 

 the basis of a method of determining the oxygen tension of arterial blood by 

 Douglas and Haldane (1912). According to Nicloux (1913, 1914), the relative 

 proportions of carboxyhaamoglobin and oxyhaemoglobin is regulated by mass action. 



The considerations with regard to adsorption from mixtures, referred to above 

 (page 622) in the case of the driving off of oxygen by carbon dioxide, apply also to 

 carbon monoxide. This gas is more strongly adsorbed than oxygen is. 



Optical Properties. Haemoglobin and its derivatives give very definite absorp- 

 tion spectra. In Fig. 193 a series of photographs is given. The fact is of 

 practical value in the colorimetric and spectro-photometric methods of estimation 

 of haemoglobin in general use. One cannot, at present, assign any significance to 

 the absorption of light from the photo-chemical point of view, except that, as we 

 saw above (page 571), the absorption of ultra-violet light has probably a pro- 

 tective function. 



Hartridge and Hill (1914) have made interesting observations on the infra-red absorption 

 of haemoglobin, comparing it with that of reduced haemoglobin and the compound with 

 carbon monoxide. They find that it is considerable in this region, which has great radiation- 

 energy, and that the absorption of carboxyhsemoglobin is only about half that of oxy- 

 haemoglobin. It is clear that determinations of the absorption in this region of the spectrum 

 would enable estimations to be made of the relative amounts of the three substances present 

 in a solution, a point of practical importance, as we shall see later in connection with the 

 oxygen tension in blood. The measurements would be made by a thermopile, as described in 

 the catalogue of Messrs Adam Hilger. This infra-red absorption is of interest in another 

 way. Light produces a change in the equilibrium between oxygen, carbon monoxide, and 

 haemoglobin, as shown by Haldane and Lorrain Smith. We have seen that, by Nernst's 

 formula (1913, p. 679), we can calculate the free energy of a reaction, if we know the 

 equilibrium constant. Therefore, we have here a photo-chemical reaction, in which light 

 energy can be stored. Hartridge and Hill calculate, from the known change of the equilibrium 

 constant, in the above reaction, what this amount of energy is, and find that it is very con- 

 siderable, in fact, much greater than that of any similar photo-chemical reaction, with the 

 exception of that of the chlorophyll system. 



Chemical Constitution. This question was discussed briefly in Chapter XIX. 

 (pages 560-561) in relation to chlorophyll, and the meaning of the iron content 

 was referred to in an earlier part of the present chapter (page 614). 



Methods of Investigation. A useful account of the methods used in the 

 determination of the degree of oxygen saturation of haemoglobin is given in detail 

 in the appendix to Barcroft's book (1914). The apparatus of Winterstein 

 (1912, 1), especially with the later improvements (1913, 2), is, in many respects, 

 very convenient in use, both for blood gas analysis and for respiratory exchange 

 of small organs. It is more fragile than that of Barcroft (see Fig. 189 above). 



THE LUNGS 



We have seen how oxygen is conveyed to the tissues, by the agency of 

 haemoglobin, in greater quantity than could be done if it were merely dissolved 



40 



