Answer
The H+ cations that combine with CL- anions to form HCL, are derived from carbonic acid molecules (H2CO3).
Work Step by Step
In the parietal cells the enzyme carbonic anhydrase catalyses the combination of carbonic dioxide and water to form carbonic acid (H2CO3). H2CO3 dissociates to give the ions H+ ad HCO3-. The parietal cells exchange HCO3 for CL- and eventually releases H+ and Cl- ions into the gastric juice.