Also known as stomach acid, gastric acid is the substance secreted in the stomach that is responsible for the chemical digestion of food. A combination of hydrochloric acid (HCl) and the salts sodium chloride (NaCl) and potassium chloride (KCl), it is produced and released by cells in the epithelium or lining of the stomach known as parietal cells. Humans consume three macronutrients in food — carbohydrates, fat, and protein. Of these, it is protein that this acid breaks down in the stomach, separating its component amino acid chains by the conversion of a digestive enzyme called pepsinogen into another enzyme known as pepsin. Gastric acid also functions to prevent harmful bacteria like E coli from flourishing in the digestive tract, as it cannot reproduce in such an inhospitable environment.
Digestion is the process by which consumed food and beverages traveling through the gastrointestinal tract are broken down through both physical and chemical means into their most basic molecular components to supply the body with calories, or energy. Examples of physical digestion include mastication, or chewing, and the churning of food by the stomach. Chemical digestion is made possible by digestive fluids like saliva and stomach acid, which either contain or activate the digestive enzymes that deconstruct the macronutrients in food. Once these nutrients are broken down, they may be absorbed by the intestines, with unneeded digestive byproducts exiting the body as waste.
Gastric acid contributes to this process by going to work on protein, typically the slowest of the nutrients to enter the bloodstream. While the breakdown of carbohydrates and fats begins with saliva in the mouth, which along with mastication converts the food into a partially digested mass known as bolus, the breakdown of protein does not occur until the bolus is swallowed, carried through the esophagus, and enters the stomach. Once it passes through the esophageal sphincter, whose job is to prevent the reflux of gastric acid back into the esophagus, the next step of chemical digestion begins.
Composed of a network of secreting glands called canaliculi, parietal cells in the epithelium, or innermost layer of stomach lining, release ions of chlorine and hydrogen. These ions combine to form hydrochloric acid and are joined by potassium and sodium ions to form gastric acid. While highly acidic when still contained by the parietal cells, this fluid achieves a pH of one to three once it reaches the lumen, or interior, of the stomach as it becomes diluted. It should be noted that the synthesis of gastric acid begins before food is even consumed, initiated by the smell, appearance, and even the expectation that food is forthcoming.
Once secreted into the lumen, gastric acid alters the acidity of the stomach, which has the effect of revealing the peptide bonds that link the protein’s amino acids in their chains, bonds that are normally protected by the organization of these chains into corrugated or folded shapes. When these bonds are exposed, gastric acid turns on pepsinogen enzymes secreted into the lumen by other epithelial cells known as chief cells, converting the pepsinogen to pepsin. Pepsin then locates the peptide bonds and eliminates them, thus separating the individual amino acids from the chain, acids that can be absorbed in the small intestine.
After the partially digested bolus leaves the stomach mixed with gastric acid and enters the small intestine via the duodenum, sodium bicarbonate (NaHCO₃) raises the pH level and neutralizes the acid so that it cannot damage the intestinal lining. A layer of mucus protects the stomach lining from acid damage. The intestinal wall lacks this protection. Neutralization by sodium bicarbonate, better known as baking soda and released by the pancreas, returns the pH of the digestive tract to approximately a seven, or neutral.