422 LECTURE XVIII. 



however, the relations are by no means so simple. The dissociation of the 

 bicarbonate in solutions of the same concentration as the serum at 37 C., 

 becomes noticeable only when the pressure of the gas upon the solution is 

 less than a few millimeters. With a pressure of 0.2 millimeter about 

 three-fifths of the total dissociable carbonic acid still remains chemically 

 combined. According to the observations of Jaquet, cited above, the car- 

 bonic acid of the plasma behaves quite differently. Complete saturation 

 is not effected with 15 millimeters of carbon dioxide pressure. The behavior 

 of the carbonic acid loosely combined in the plasma, therefore, cannot be 

 explained by its relations to bicarbonates and monocarbonates. The fact 

 that by means of the air-pump more than half of this carbonic acid may 

 be expelled from the plasma, speaks, more than anything else, against any 

 such assumption. Inasmuch as we know of no other compounds in the 

 plasma which would be capable of uniting with carbon dioxide to any 

 considerable extent, we are forced to believe that other weak acids are 

 present in the plasma which are constantly striving to unite with the 

 alkali. Sertoli 1 long ago looked for such acids, and considered as such 

 the protein substances of the plasma, especially the globulins. To-day 

 there is no longer any doubt that these protein substances are actually 

 present in the form of alkali salts in serum. They are driven out of these 

 compounds if there is an excess of carbonic acid present. N. Zuntz and 

 A. Lowy 2 have shown this assumption to be true in a very convincing 

 manner. They found that the amount of diffusible alkali in the serum 

 increased by conducting carbon dioxide into it. This is to be attributed 

 to the fact that as the carbonic acid is forced into the serum, the alkali 

 albuminates which are not diffusible are decomposed, and alkali carbonates 

 which are capable of passing through the membrane are formed in their 

 place. 



The next point to be decided is whether the alkali contained in the 

 serum is entirely combined with albumin when the partial pressure of the 

 carbon dioxide gas is equal to 0, or whether an excess of alkali is present ? 

 If, other than alkali albuminates, there were no other alkali salts of weak 

 acids present in the serum, then it would be expected that if the alkali 

 were completely combined with the protein (i.e., when the partial pressure 

 of the carbonic acid gas was zero), all of the carbon dioxide would 

 have been driven out of the plasma. This is not the case, as E. Pfliiger 3 has 

 shown. In one experiment he found 4.9 per cent by volume, and in 

 another 9 . 3 per cent of carbon dioxide which remained in the plasma, and 



1 Sertoli: Hoppe-Seyler, Medizin-chem. Untersuch. Berlin, 1868, p. 350. Cf. N. 

 Zuntz: Hermann's Handbuch der Physiol. Bd. 4, 64 (1882). Torup: Die Kohlensaure- 

 spannung des Blutes, Kopenhagen, p. 36 (1887). Kurt Brandenburg: Z. klin. Med. 45, 

 H. 3 and 4. 



2 Pfliiger's Arch. 58, 511 (1894). 



3 Die Kohlensaure des Blutes, p. 11, Bonn. 1864. 



