DIGESTION 



313 



The hydrochloric acid of gastric juice is made from blood which has 

 an approximately neutral reaction. The hydrogen ion of hydrochloric 

 acid comes from carbonic acid and the chloride ion originates from 

 sodium chloride. These ions are selectively absorbed from the blood 

 by the parietal cells, and the hydrochloric acid thus formed is secreted 

 into the stomach. The loss of hydrogen ions from the blood is evidenced 

 by an increase in the alkalinity of the blood which has passed through 

 the gastric mucosa of the stomach during a period of active hydrochloric 

 acid secretion. The precise 'mechanism by which hydrogen and chloride 

 ions are concentrated by parietal cells to form an acid from the almost 

 neutral blood has not been completely elucidated. It should be realized 

 that a great deal of energy is required to raise the hydrogen ion concen- 

 tration from 4X lO'^M (pH 7.4) in blood to O.IQM (pH 0-1), the 

 hydrogen ion concentration of parietal secretion. This transformation 

 constitutes an approximate 4,000,000 fold increase in the concentration 

 of H + . 



The relationship of some components of the blood to the secretion of 

 hydrochloric acid by the parietal cells is pictured in the following scheme. 



Incoming Blood 



Outgoing Blood 



COo + HoO 



H2CO3 



H+ + HCO3- 

 XaCl 



CI- + Xa+ 



(Parietal membrane) 



-\ 



-^ HCOr 



+ UXaHCOa 



:o+ 



Na 



J 



H+ + Cl- 



:^HC1 



(Appears in parietal cell secretion) 



The high acidity of gastric juice has a bactericidal effect on the micro- 

 organisms ingested with the food and is ideal for the action of some 

 of the digestive enzymes present in this fluid. Most of the enzymes of 

 gastric juice work best at pH 2-4. 



The principal proteolytic enzyme of gastric juice is pepsin. It attacks 

 proteins and reduces them to smaller fragments such as proteoses, pep- 



