534 



THE RESPIRATORY EXCHANGE 



It will be necessary to consider this question of the pressure 

 of carbon dioxide in the blood in a later chapter, when the causes 

 of the gaseous exchange are discussed : the nature of the chemical 

 combinations into which carbon dioxide enters must now be con- 



O 10 20 30 40 50 6O 7O 8O 



1OO HO 1ZO 13O 



FIG. 30. Pressure of Carbon Dioxide in Blood at 38 (Bohr). 

 Jaquet's data are marked by o, Bohr's data by x. 



sidered, and in the first place those found in the plasma. For 

 many years it was held by physiologists that one of the essential 

 factors in the combination of carbon dioxide was disodium hydrogen 

 phosphate, Na.,HP0 4 , which can combine with carbon dioxide and 

 form sodium bicarbonate and dihydrogen sodium phosphate. 



Na 2 HP0 4 + C0 2 + H 2 = NaHCO s 



Experiments, however, have shown that the quantity of phos- 

 phoric acid in the ash of serum is so small that, if allowance be 

 made for that derived from lecithin and nucleo-albumins, the 

 amount is quite insufficient to play any important part in the 

 absorption of carbon dioxide. There are, however, present in 

 blood greater amounts of other alkalies. Thus, according to 

 Bunge, the ash of 1000 grm. of dog's serum contains 4-341 grm. 

 of sodium, of which 3*463 grm. are sufficient to combine with the 

 chlorine ; the remaining 0-878 grm. of sodium could combine 

 with 0-623 grm. of carbon dioxide (316 c.c. at and 760 mm.) 



