424 LECTURE XVIII. 



absorption. This part is chemically combined, and in fact, chiefly with 

 the pigment, hemoglobin. This may effect the absorption of carbon 

 dioxide in two ways. In the first place, the globin in it and the remaining 

 proteins of the blood, may strive to combine with the alkali, as does the car- 

 bonic acid. On the other hand, the hemoglobin itself may unite directly 

 with the carbon dioxide. The first way in which the hemoglobin influences 

 the absorption of the carbon dioxide is perfectly analogous to what we 

 have just been discussing with regard to the plasma. Here, also, the union 

 between the hemoglobin and the alkali will not be dissolved until the pres- 

 sure of the carbon dioxide has reached a certain value. Thus N. Zuntz 1 

 found the compound between the hemoglobin and alkali was not decom- 

 posed appreciably, until the pressure of the carbon dioxide was greater 

 than 70 millimeters. This benefits the organism only in time of excep- 

 tional need. 



We shall now consider the nature of the chemical union between the 

 carbonic acid and the hemoglobin itself. We have seen that hemoglobin 

 combines with oxygen, and that this property is peculiar to that part of 

 the molecule which contains the iron, while the globin participates indi- 

 rectly in the reaction only in as much as the union of the globin with the 

 hematin brings forth relations which change the firm state of combination 

 between oxygen and hematin into one which is more readily dissociable. 

 It is conceivable that the carbon dioxide unites with the same part of 

 the hemoglobin molecule that oxygen does. We do in fact know of gases 

 of which this is true, as, for example, carbonic oxide (carbon monoxide) . 

 One volume of the latter gas replaces one volume of oxygen. 2 This car- 

 bonic oxide compound with hemoglobin is also dissociable. The carbon 

 monoxide may be replaced by oxygen again. This takes place when the 

 partial pressure exerted by the oxygen exceeds that of the carbonic oxide. 3 

 The fact that the carbonic oxide actually combines at the same place as 

 the oxygen was shown to be very probable by Hoppe-Seyler, 4 who showed 

 that it was combined in the iron-containing radicle of the molecule. 

 Now it is well known that carbonic oxide has a poisonous effect; and appar- 

 ently this is due to the fact that it replaces the oxygen in its combination 

 with the hemoglobin, so that it seriously affects the supply of oxygen for 

 the tissues. Such an action is unknown in the case of carbon dioxide. It 

 is also a priori hardly probable that oxygen and carbonic acid should each 

 strive for possession of the hemoglobin molecule. It has also been shown 



1 Zentr. med. Wiss. 6, 529 (1867). 



2 G. Hiifner: Arch. Anat. Physiol. 1895, 209. Hiifner and Kiilz: J. pr. Chem. 28, 

 256 (1883) ; and Hiifner: ibid. 30, 68 (1884). 



3 Recently Hiifner and Kiister have undertaken experiments to determine the rela- 

 tions by weight in which hemochromogen combines with carbonic oxide. Arch. Anat. 

 Physiol. 1904, 387. 



* Z. Physiol. Chem. 13, 477 and 493 (1889). 



