THE CHEMISTRY OF RESPIRATION 



1117 



The upper curve gives the dissociation of a sodium bicarbonate solution, 

 and the lower that of blood at varying pressures of C0 2 , the bicarbonate 

 being of the strength which we have assumed to exist in blood, viz. 

 4-83 x 10~ 2 N. A solution of pure bicarbonate has a very small tendency 

 to dissociate and is completely stable at a pressure of 4 6 mm. Hg. C0 2 . 

 If the pressure is reduced below this the sodium bicarbonate slowly dis- 

 sociates, but the dissociation never passes beyond the formation of sodium 

 carbonate, so that in a complete vacuum sodium bicarbonate will give off 

 only half the total carbonic acid that it contains. 



120 



I2O MM. 



FIG. 513. Dissociation curve of a 4-83 X 1Q- 2 N solution of sodium 

 bicarbonate (upper curve) compared with that of human blood, at 

 37C., at varying tensions of CO 2 . (From PABSONS. ) 



Blood on the contrary at a C0 2 pressure of 50 mm. Hg. is only about 

 half saturated with C0 2 , and to a vacuum gives off the whole of its content 

 in carbonic acid, so that no further evolution is attained on the addition 

 of acid. Are there any conditions in the plasma or in the whole blood 

 which may, so to speak, loosen this attachment of carbonic acid to sodium 

 in the bicarbonate, so as to enable this gas to be given off more rapidly on 

 exposure to diminished pressure? 



We may artificially make a fluid which behaves to carbon dioxide in 

 the same way as blood, by mixing together sodium carbonate and sodium 

 hydrogen phosphate Na 2 HP0 4 . From such a mixture the whole of the 

 carbon dioxide may be given off when exposed to a vacuum. On the other 

 hand, a large amount of carbon dioxide will be taken up with a very small 

 difference in tension of the gases. The behaviour of the mixture is due to 

 an interaction which occurs between the acid radicals P0 4 and C0 3 . When 



