Protons and the Parietal Cell
Stomach acid is primarily composed of a substance called hydrochloric acid, which is made by cells in the stomach called parietal cells. According to Colorado's State Pathology Department, stomach acid is notable for its high concentration of molecules called hydrogen ions, which can also be called protons. These hydrogen ions are made from the breakdown of water into a hydrogen ion and a hydroxide ion. The parietal cells then get rid of the hydroxide ion by using an enzyme called carbonic anhydrase. Carbonic anhydrase takes hydroxide ions and combines them with carbon dioxide to make bicarbonate. The net result of these chemical reactions is that parietal cells turn water and carbon dioxide into hydrogen ions and bicarbonate.
Proton Pump
Once the parietal cell has made the protons, it has to move them into the stomach. Because the parietal cell needs to move a lot of protons into an area that already has a high proton concentration as well, the parietal cell has to use a molecular pump, which pumps protons out of the parietal cell and into the stomach and pumps potassium ions back into the parietal cell. This pump is powered by a molecule called Adenosine-5'-triphosphate (ATP), which is a high-energy molecule that cells use for energy. While the protons are being pumped out, the bicarbonate ions that are formed along with the protons also need to be pumped out of the cell to keep the parietal cell from becoming too alkaline (the opposite of acidic). As a result, the bicarbonate is secreted into the blood.
Regulation
Colorado State's Pathology Department explains that there are three main molecules that stimulate the production of stomach acid. One of these molecules is called gastrin, which is made by G cells in the stomach when food is present. Not only does gastrin cause more stomach acid to be made, it also causes cells in the stomach called enterochromaffin-like cells to make histamine. Histamine also stimulates the secretion of stomach acid. Finally, stomach acid can be made in response to acetylcholine, which is a chemical that nerves secrete to let the brain tell the stomach to make more acid. All of these three chemicals cause a slight increase in the production of stomach acid, but when all three are present, they cause an increase in the activity of the parietal cells.
