TRANSPORT OF CARBONIC ACID 103 



NaHCOg is not dissociated. Nor is there conclusive 

 evidence for the behef that CO2 combines with hsemoglobin. 

 There are two mechanisms whereby the blood accom- 

 modates itself to varying amomits of COg. The first is due 

 to the proteins of the -plasma. A protein, by virtue of the 

 H atom of the COOH group, is an acid and is capable of 

 combining \vith sodium according to the amount of sodium 

 available, this in turn being determined by the amount 

 of sodium required to combine with COg. The sodium, 

 in other words, shifts to and fro between the CO2 and the 

 proteins, the direction of the movement being determined 

 by the amount of CO2 present. Here is a diagrammatic 

 representation — ■ 



CO. CO^ CO. Protein Protein Protein Protein 



Na Na Na Na Na 



CO2 CO. CO,, CO. CO. Protein Protein Protein Protein 



Na Na Na Na Na 



Since the proteins are very weak acids, the amount of free 

 protein does not affect the H. ion concentration. 



The second method is the interaction between plasma and 

 corpuscles already described (p. 19). When CO2 is added to 

 the blood, CI ions pass from the plasma to the corpuscles, 

 thus allowing sodium to enter into combination with the 

 acid. 



THE PASSAGE OF OXYGEN INTO THE BLOOD 



From the alveoh of the lungs oxygen gains the blood by 

 passing through the flattened cells of the lung epithehum, 

 across the lymphatic space and through the endothehal 

 wall of the capillaries. Is this process one of diffusion or 

 is it due to active secretion of oxygen into the blood by 

 the lung epithehum? If the process is to be explained 

 by diffusion it is necessary to show that the tension of 



