xi EESPIEATOKY EXCHANGES 385 



be dissociation of the haemoglobin of the erythrocytes from the 

 oxygen, which diffuses in the plasma to re-establish equilibrium 

 between the tension of the oxygen in the plasma and the corpuscles. 



VII. The carbonic acid of the blood is also for the most part 

 in chemical combination, and to a minimal extent in solution. 

 This is proved by the fact that the coefficient of absorption of this 

 gas in water, at 37 C., is about 0'57, while on the other hand we 

 have seen that arterial blood only contains 34 and venous blood 42 

 vols. per cent. Unlike oxygen, however, which enters into com- 

 bination only with haemoglobin, carbonic acid unites chemically 

 with many substances, both of the plasma and of the corpuscles. 



Among the substances capable of holding the carbonic acid of 

 the blood in readily dissociable forms, great stress was formerly 

 laid upon sodium carbonate, which as a base (see p. 132) abounds in 

 the ash of plasma. The phenomena of electrolytic dissociation of 

 the solutions of this salt have, however, demonstrated that it can 

 only be of very secondary importance in the chemical combination 

 of the carbonic acid of the plasma, by converting it into bi-carbonate. 

 In fact, according to the researches of Bohr, a very dilute solution 

 of 0*15 per cent sodium carbonate becomes almost saturated at a 

 pressure of only 10 mm. Hg of carbonic acid, while on raising the 

 pressure to 120 mm. there is no appreciable increase in the amount 

 fixed or dissolved. Hence it is evident that sodium carbonate is 

 incapable of fixing more than a minimal amount of the carbonic 

 acid of the blood. 



The alkaline phosphates of plasma, which are capable of con- 

 version into acid phosphates by association with carbonic acid, were 

 again erroneously credited with too much importance (Fernet). 

 We saw in fact (p. 139) that the main part of the phosphoric 

 anhydride found in the ash of plasma is derived from combustion 

 of the lecithin and nucleo-albumins, and that normal plasma con- 

 tains only the merest trace of sodic phosphate (Sertoli). 



On the other hand, according to the observations of Setschenow 

 and Torup, maximal importance in the fixation of carbonic acid 

 must be assigned to the globulins of the serum, which, by acting 

 as weak acids, are able to combine with the alkalies of the blood. 

 When the tension of carbonic acid increases, the globulins of the 

 alkalies are dissociated, and combine with the carbonic acid to 

 form carbonates ; when, on the contrary, the tension of carbonic 

 acid falls the globulins are again associated with the alkalies, 

 leaving the C0 2 in the free state. 



Since carbonic acid is also found in the corpuscles of the blood 

 in a readily dissociable form, it is probable that the combinations 

 of the globulins with the alkalies exercise the same office in the 

 corpuscles as in the plasma, in the dissociation of carbonic acid. 

 It should, however, be noted that the absorption of the latter, as 

 effected by the corpuscles, is, in comparison with absorption in the 



VOL. i 2 c 



