Stomach acid is an important part of the digestive system. Stomach acid serves to help break down food, but it also can be used to activate digestive enzymes, which are special proteins that help break down some of the components of food. These enzymes are only activated by acid, so the secretion of stomach acid is a carefully controlled and regulated process.
Stomach Acid Composition
Stomach acid is an acid solution that is secreted by cells in the lining of the stomach. Stomach acid has a pH between one and two, and it is essentially a solution of hydrochloric acid (which is, in chemical terms, a strong acid) and large amounts of sodium chloride and potassium chloride, which are salts and play no role in gastric acid's acidity. Stomach acid is produced by the parietal cells.
Mechanism
One of the main components of stomach acid is positively charged hydrogen ions. Because these ions have to be moved into an area where they are already highly concentrated, it requires energy to move them into the stomach. This requires the parietal cells of the stomach to use a molecular pump (which uses energy) to pump the hydrogen ions into the stomach.
Neural Stimulation
The parietal cells can be stimulated to make stomach acid via a number of different sources. One way in which the parietal cells can become told to make stomach acid is via nerve impulses. Specifically, the vagus nerve and other nerve branches provide innervation to the parietal cells. These nerves are part of the parasympathetic nervous system, an automatic system that, among other things, stimulates food digestion. These nerves activate the parietal cells by releasing a chemical called acetylcholine.
Histamine
Another chemical that can stimulate parietal cells is called histamine. Histamine is released by a number of different cells and activates the hydrogen ion pumps in parietal cells. One class of medication that can be used to treat acid reflux and ulcers blocks the effects of histamine on parietal cells.
Gastrin
Stomach acid production can also be stimulated by other parts of the digestive system. Other parts of the digestive system can signal to the stomach that more stomach acid needs to be produced by secreting a chemical called gastrin. Gastrin activates the parietal cells and causes them to secrete more stomach acid.
