338 RESPIRATION 



considered as by any means settled. The difficulty lies just here, that whereas 

 oxygen is evidently present only in hemoglobin, carbon dioxide is united with 

 several different substances. 



The researches of Paul Bert, Zuntz, Setchenow, and others have made it 

 perfectly evident that carbon dioxide is present for the most part in dissociable 

 compounds, the existence of which depends upon the prevailing partial pres- 

 sure of C0 2 . In accordance with what was said regarding the combination 

 of oxygen with haemoglobin, it is evident also that a certain quantity of free 

 C0 2 in the blood must be present in physical combination (cf. page 336). 

 The coefficient of absorption of carbon dioxide in water at 37 is 0.569. The 

 dissociable compounds are found both in the plasma and in the corpuscles. 



Of the substances in the blood with which carbon dioxide can be combined, 

 sodium bicarbonate NaHC0 3 is likely to be thought of first. The phenomena 

 of dissociation in solutions of this salt show however that it cannot play any 

 great part in this connection; for according to Bohr a 0.15-per-cent solution 

 of sodium bicarbonate under a pressure of 0.6 mm. Hg. takes up eighty per 

 cent of the total quantity of carbon dioxide which can be taken up under a 

 pressure of 120 mm., and at a pressure of only 10 mm. it is almost com- 

 pletely saturated. 



Again great importance has been ascribed to the phosphates in the com- 

 bination of C0 2 , since it was supposed from analyses of the blood that the 

 plasma contained large quantities of these salts. But it has been shown that 

 the phosphorus found in the ash is primarily a constituent of lecithin and 

 nucleoalbumin, and occurs only in traces as Na 2 HP0 4 . 



The globulin-alkali compounds, on the other hand, appear to be of far 

 greater importance for the combination of C0 2 in the blood. The globulins 

 play the part of weak acids and enter into saltlike combinations with the 

 alkalies of the blood. They can be replaced from these compounds by carbon 

 dioxide and can themselves in turn replace the carbon dioxide. 



The significance of this fact will be more apparent from the following: 



If two acids of different avidity represented respectively by a and b be pres- 

 ent in a solution of a basic substance, they divide the basic substance between 

 them in the ratio of a/b. Under the influence of equal mass equivalents of the 



two acids and of the base, - equivalents of the one acid, and of the 



a-\- b a-\-b 



other will unite with the base. But if the substances are not present in equal 

 mass, the distribution of the base between the two acids will depend upon the 

 relative masses of the two, so that the acid present in the greater quantity rela- 

 tively, even if its avidity is weaker, will get the greater quantity of the base. 



Applied to the problem before us, this would mean that, if the mass of car- 

 bon dioxide, or more properly its tension in the plasma, is high, the globulin 

 will be forced out of its alkali compound. If, however, the blood comes into 

 such relations that the carbon dioxide tension falls, the globulins again suc- 

 ceed to their rights and the carbon dioxide leaves the alkali (Torup). 



As already observed, carbon dioxide occurs also in the blood corpuscles 

 in the form of dissociable compounds. It is very probable that the globulin- 

 alkali compounds of the blood corpuscles act in the same way as those of the 



