416 LECTURE XVIII. 



moreover, very questionable whether hemoglobin itself is a simple substance. 

 The component containing iron is constant in its composition, but the 

 relation between this and the globin (the protein constituent) , or, in other 

 words, the numberof globin molecules which unite with the hemochromogen, 

 varies in different cases. It is necessary to mention this fact in this con- 

 nection, because it is unquestionably true that differences in the results 

 obtained by investigators are due, to some extent at least, to the fact that 

 the results obtained by working with hemoglobin solutions have been 

 applied directly to the absorption by the blood. 



Hemoglobin itself consists of a protein, globin, and another constituent, 

 hemochromogen, 1 which contains iron; it is the hemochromogen alone that 

 unites with oxygen. This is shown by the fact that hemochromogen absorbs 

 as much oxygen from the air as an equivalent amount of hemoglobin. 

 When hemochromogen is oxidized to hematin, the hemoglobin of the blood 

 becomes oxyhemoglobin. Whereas the oxygen combined in hematin can- 

 not be removed by means of an air-pump, this is not the case with oxy- 

 hemoglobin itself. This fact is of great importance for the understanding 

 of the transportation of oxygen by the blood and its giving up of the same 

 to the tissues. Oxyhemoglobin belongs to the class of compounds which 

 are said to be dissociable. Before we go into further particulars concern- 

 ing this transportation of oxygen by the blood and the subject of internal 

 respiration, we must make perfectly clear what are the conditions in the 

 animal organism upon which the dissociation of the oxyhemoglobin de- 

 pends. We have already mentioned the fact that the blood-plasma contains 

 absorbed oxygen. The amount present is, corresponding to the gas laws, 

 relatively small. It is perfectly clear that it follows the laws of gas absorp- 

 tion. First of all, the amount of this gas must be in equilibrium with the 

 alveolar air. On the other hand, this absorbed oxygen, in its transporta- 

 tion to the various tissues, must necessarily constantly seek to be in 

 equilibrium with the pressure of the oxygen in these tissues, and, likewise, 

 in accordance with the well-defined gas laws. Now, as we shall soon see, 

 the tissues are constantly consuming oxygen and forming carbon dioxide 

 therefrom. For this reason the pressure of oxygen in the tissues is kept 

 lower than that of the absorbed (or dissolved) oxygen in the blood. Hence 

 oxygen is constantly entering these tissues from the blood. There is no 

 doubt at all that, in the first place, this absorbed oxygen is given up. Now 

 in proportion as this absorbed oxygen is given up by the blood, oxyhemo- 

 globin becomes dissociated, i.e., begins to give up its oxygen to the plasma. 

 If this conception be correct, it must be possible to remove eventually 

 all the oxygen from the blood by means of the air-pump, even at 

 low temperatures, i.e., without the aid of heat, which itself tends 



1 Cf. Lecture VII, p. 141. 



