86 RESPIRATION 



protein solutions will combine with CO 2 . A considerable excess 

 of acid or alkali must be added to a neutral protein solution before 

 a marked acid or alkaline reaction is reached. The protein acts as 

 a "weak," or very slightly ionized, acid, such as carbonic acid, and 

 likewise acts as a correspondingly weak alkali, since the protein 

 molecule possesses both acid and alkaline affinities. It is thus, like 

 carbonic acid, or any other weak acid, or weak alkali, a buffer 

 substance, which prevents any abrupt change from acid to alkaline 

 reaction or vice versa. Not until all the CO 2 combined in a solu- 

 tion of carbonate has been liberated by acid is there a sudden de- 

 velopment of acid reaction, or so long as any free CO 2 is present 

 of strong alkaline reaction. The CO 2 acts as a buffer substance on 

 the alkaline side only, whereas protein is capable of acting on 

 either side of the neutral point. In the living body, however, blood 

 is always a little alkaline, so that the combination of CO 2 with 

 proteins does not come into account. 



We can now see a reason why blood should act towards CO 2 as 

 it does in the living body and in the vacuum pump. The total 

 alkali in the blood is combined, partly with strong acids, such as 

 HC1, partly with carbonic acid, and partly with protein com- 

 pounds ; partly also, perhaps, with other substances capable of 

 acting, like the proteins, as very weak acids. In the living body, 

 however, free carbonic acid is always present, and the mass in- 

 fluence of the free carbonic acid prevents part of the protein from 

 combining with alkali, while the protein in a similar manner 

 keeps out the carbonic acid. We have thus a chemical system 

 which is disturbed at once by any variation in the concentration 

 of free carbonic acid present, i.e., by any variation in the partial 

 pressure at which the blood is saturated with CO 2 . When the 

 pressure of CO 2 falls, more of the proteins are at once enabled to 

 take the place previously occupied by the carbonic acid in the 

 chemical combinations which constitute the system ; and vice versa 

 with a rise of CO 2 pressure. In the vacuum pump the CO 2 pressure 

 is reduced to zero, since, although the total pressure in the vacuum 

 chamber of the pump is, owing to aqueous vapor, always above 

 zero, the CO 2 is carried off in the stream of aqueous vapor passing 

 away. To recover the whole of this CO 2 in the same gaseous form, 

 however, a perfect and dry vacuum in the receiving chambers of 

 the pump is needed. Since the CO 2 pressure is zero the whole of the 

 CO 2 in combination is expelled by the mass influence of protein act- 

 ing as an acid. Pfliiger showed that even when a moderate amount 

 of sodium carbonate is added to blood, the additional CO 2 in the 



