614 PRINCIPLES OF GENERAL PHYSIOLOGY 



Now, suppose that we expose blood, or a solution of hemoglobin, to oxygen at 

 its pressure in the air and shake together until no more oxygen is taken up. We 

 find that, even when exposed to oxygen at a higher pressure, no more is taken 

 up. At least, this is what is usually held to be the case, but there are very 

 few experimental determinations which show this fact directly. At all events, it 

 is practically " saturated," as shown by the form of the curve which we shall learn 

 to call the "dissociation-curve." 



Barcroft has recently made some determinations of the amount of oxygen taken up by 

 blood exposed to a gaseous mixture of 8> per cent, oxygen and lo per cent, nitrogen, and finds 

 the following percentage degrees of saturation as compared with that regarded as complete : 

 102, 99, 98, and 97 in four experiments. These values were corrected for the gas physically 

 dissolved, and point to a true saturation point. They were kindly communicated to me by 

 the experimenter. 



Next, let us take haemoglobin, which has been saturated with oxygen at the 

 pressure in which it exists in the atmosphere, and compare the amount of its 

 content in iron with the oxygen contained. It has been satisfactorily proved by 

 Peters (1912), in very careful and accurate work, that the amount of oxygen taken 

 up corresponds to that required to convert the iron into FeO . Of course, this 

 does not mean that the oxygen is actually combined in this way, as sometimes 

 appears to be thought. Such a peroxide does not seem to be known and the iron 

 is united also in organic combination. A trivalent iron might be united to two 

 atoms of oxygen in peroxide form and the third valency attached to the organic 

 group, but such a combination does not agree with the formula given by Kilster 

 (1912, p. 469). Too much stress must not be laid on this point, since it is difficult 

 to see what is the function of the iron, except to combine with oxygen. It is to 

 be remembered that the iron in hemoglobin is not in such a form as to be electro- 

 lytically dissociated, and that it gives none of the reactions of iron salts. All that 

 we are really justified in saying is that, when saturated with oxygen, each molecule 

 of hemoglobin contains two atoms of oxygen to each atom of iron, or, in other 

 words, that each molecule of hemoglobin takes up the same definite amount of 

 oxygen. The work of Laidlaw (1904), however, tends to show that the iron is in 

 different combination in reduced hemoglobin to that in which it is in oxyhemo- 

 globin, since the iron-free derivative, hematoporphyrin, is easily obtained by the 

 action of acid on the former, while, under the same conditions, hematin is obtained 

 from the latter ; that is, the iron is not split off. 



But, while there is no doubt that the ratio given holds for hemoglobin saturated 

 with oxygen at its pressure in the atmosphere, say 160 mm. of mercury, it is 

 a curious fact that in the presence of salts, as in the curve on p. 45 of 

 Bancroft's book (1914), the course of the curve has the appearance of going 

 beyond the ordinate marked 100 per cent, saturation. Is it possible that the 

 saturation point is assumed to be that of the asymptote of the rectangular 

 hyperbola deduced by the application of the law of mass action? As we shall 

 see presently, this is one of the points that remains to be proved. It is quite 

 possible that it will be found to be the case that complete saturation is attained 

 at 160 mm. oxygen tension, but if it should be found that, under higher tensions 

 in the presence of salts, more oxygen can be taken up than that corresponding to 

 one molecule of oxygen to one atom of iron, the fact that this obtains at 160 mm. 

 tension must be due to chance, certainly an unlikely possibility. Thus we have 

 met with the first puzzle, but a more difficult one will be found immediately. 



There are one or two interesting problems with regard to the function of iron in haemoglobin 

 which have not. so far as I know, been investigated. Hsematin, which is hsemoglobin minus 

 its protein constituent, but containing iron in the same form of combination, loses oxygen by 

 the action of reducing agents and becomes ha?mochromogen. This latter takes up oxygen 

 from the air again. Now, has hjemochromogen the property which hemoglobin has, as we 

 shall see presently, of taking up different amounts of oxygen from different pressures? It 

 would appear that it has not, since the oxygen of hsematin cannot be removed by the air- 

 pump. In fact it behaves as a chemical compound should, according to the phase rule, as 

 we shall see. Methsemoglobin, again, contains iron in organic combination, but does not 

 give up its oxygen to a vacuum. It has been stated that a protein, obtained from yolk of 

 egg by Bunge, contains iron. 4s it capable of taking up oxygen 1 The behaviour of dry 

 haemoglobin to oxygen is another point of interest. 



