Human digestion [local] is extracellular and pH adjustments are made by the parietal cells in the stomach and the pancreas for the duodenum. The stomach is the site of much of the initial chemical digestion. The parietal cells secrete HCl into the stomach. The mechanism for acid [local] secretion involved the enzyme carbonic anhydrase and active transport. Over-secretion of HCl can be reduced by use of H2 receptor antagonist and proton pump inhibitors. Ulcers [local] are thought to be caused by too much acid in the stomach, Helicobacter pylori, and lifestyle.

The pancreatic [local] duct cells secrete HCO₃^- and enzymes into the duodenum. The production of HCO₃^- is to raise the pH of chmye coming from the stomach. Production of bicarbonate is also linked to carbonic anhydrase activity. Proteases, lipases and amylases all require a higher pH environment than pepsin.

Bile [local] is secreted from the gall bladder. Bile is an emulsifying agent breaking fat droplets into smaller units and in the process increasing the surface area for the action of pancreatic lipase.