CARBONIC ACID COMBINATION 593 



alkali carbonate, and as such is subject to hydrolytic disso- 

 ciation in accordance with the law of mass action : Na 2 C0 3 + 

 H 2 C0 3 ^2NaHC0 3 . While from a solution of bicarbonate 

 of the same concentration as in the blood only about one 

 quarter of the total carbonic acid can be withdrawn even by 

 pumping all day long, if blood be subjected to the vacuum 

 of a pump the whole amount of carbonic acid is freed within 

 but a few hours. This remarkable feature is explained by 

 the fact that there are substances of an acid character con- 

 tained in the blood which displace the carbonic acid with 

 the assistance of the vacuum. Further analysis of this 

 phenomenon has shown that the carbonic acid is withdrawn 

 more slowly from the serum alone than from the general 

 blood: the latter therefore contain substances manifesting 

 acid characteristics in higher degree. These substances of 

 acid character with which we are here concerned are the hae- 

 moglobin and other proteins of the blood. We may conceive 

 that the carbonic acid and protein of acid type, and other 

 acids which gain access to the blood, as lactic acid, are in 

 competition for the possession of the blood alkali and ap- 

 portion it among themselves in accordance with the law of 

 mass action. But the situation is still more complicated by 

 the fact that the proteins have the dual nature of acids and 

 bases ; and not only may be capable of combining alkali to 

 their carboxyl, but may be capable of fixing acids to their 

 ammonia rests, among them carbonic acid. The two proc- 

 esses may go on at the same time; but it should be added 

 that a relatively high carbonic acid pressure is required in 

 order to separate haemoglobin from its alkaline combina- 

 tions, but that combination between haemoglobin and car- 

 bonic acid can take place even where the carbonic acid pres- 

 sure is low. This last feature is apparently of the greater 

 physiological significance. 



What is the nature of the combination of carbonic acid 

 with haemoglobin? While, as has been seen, the intake of 

 oxygen into haemoglobin is markedly influenced by the pres- 



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