OXYGEN. 



425 



that the carbon dioxide is taken up quite independently of the union of 

 hemoglobin with oxygen. 



This is made very clear by the experiments of Bohr. 1 He showed that 

 the presence of oxygen had no apparent effect upon the amount of carbonic 

 acid absorbed under different pressures. The hypothesis is also supported 

 by the fact that the transformation of hemoglobin into methemoglobin 2 

 does not in any way affect its combination with carbon dioxide, 3 while that 

 with oxygen is disturbed. It seems probable that the carbon dioxide is 

 not combined with the hematin part of the molecule, but rather with the 

 globin. This appears the more probable since M. Siegfried 4 has recently 

 shown that carbonic acid is associated (i.e., chemically bound) by the 

 action of amino acids and protein substances. From such compounds it 

 is very easy to set the carbonic acid free again; i.e., dissociate it. From 

 glycocoll, for example, a carbaminoacetic acid is formed. From the 

 amphoteric amino acid a relatively strong dibasic acid results. 5 The 

 carbamino acids correspond to the general type : 



H 



R N 



| X COOH. 

 COOH 



The union of carbonic acid with globin is also dependent upon the pres- 

 sure of the gas, especially when this is low. This is shown by the following 

 table. It gives the amount of carbonic acid chemically combined per gram 

 of hemoglobin at different gas pressures of carbonic acid. The concentra- 

 tion of the hemoglobin solutions amounted to 2.69 per cent; the temper- 

 ature was 38 C. 



1 Zentr. Physiol. 4, 49 and 253 (1890); and Skand. Arch. Physiol. 3, 47 and 64 (1892). 



2 See Lecture XXIV. 



8 Skand. Arch. Physiol. 8, 363 (1898). 



4 Z. Physiol. Chem. 44, 85 (1905) ; 46, 490 (1905). See Lecture XI, p. 235. 



6 It is evident that such compounds may be formed in the tissues, and that in this 

 way any momentary excess of carbonic acid can be combated. Thus, the cells can 

 assist the oxidation processes. It is also possible that the carbonic acid assimilation 

 on the part of plants, as Siegfried has suggested, may also take place through the for- 

 mation of carbaminic acids, and that the latter, and not the carbonic acid, are reduced. 



