gastric acid gastric acid gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. with a ph between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids of proteins. gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated ph. these cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. the pancreas further produces large amounts of bicarbonate and secretes bicarbonate through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract. determinants of gastric acid secretion the active components of gastric acid is hydrochloric acid. often simplistically described as hydrochloric acid, these species are produced by parietal cells in the gastric glands in the stomach. the secretion is a complex and relatively energetically expensive process. parietal cells contain an extensive secretory network (called canaliculi) from which the "hydrochloric acid" is secreted into the lumen of the stomach. the ph of gastric acid is 1.5 to 3.5 in the human stomach lumen, a level maintained by the proton pump h+/k+ atpase. the parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of ph in the blood, known as an alkaline tide. the highly acidic environment in the stomach lumen degrades proteins (e.g., food). peptide bonds, which comprise proteins, are labilized. the gastric chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen, and in infancy rennin). the low ph activates pepsinogen into the enzyme pepsin, which then aids digestion by breaking the amino acid bonds, a process called proteolysis. in addition, many microorganisms are inhibited or destroyed in an acidic environment, preventing infection or sickness. secretion a typical adult human stomach will secrete about 1.5 liters of gastric acid daily. gastric acid secretion is produced in several steps. chloride and hydrogen ions are secreted separately from the cytoplasm of parietal cells and mixed in the canaliculi. gastric acid is then secreted into the lumen of the gastric gland and gradually reaches the main stomach lumen. the exact manner in which the secreted acid reaches the stomach lumen is controversial, as acid must first cross the relatively ph-neutral gastric mucus layer. chloride and sodium ions are secreted actively from the cytoplasm of the parietal cell into the lumen of the canaliculus. this creates a negative potential of between −40 and −70 mv across the parietal cell membrane that causes potassium ions and a small number of sodium ions to diffuse from the cytoplasm into the parietal cell canaliculi. the enzyme carbonic anhydrase catalyses the reaction between carbon dioxide and water to form carbonic acid. this acid immediately dissociates into hydrogen and bicarbonate ions. the hydrogen ions leave the cell through h+/k+ atpase antiporter pumps. at the same time, sodium ions are actively reabsorbed. this means that the majority of secreted k+ (potassium) and na+ (sodium)ions return to the cytoplasm. in the canaliculus, secreted hydrogen and chloride ions mix and are secreted into the lumen of the oxyntic gland. the highest concentration that gastric acid reaches in the stomach is 160 mm in the canaliculi. this is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. the lowest ph of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a ph of between 1 and 3. there is a small continuous basal secretion of gastric acid between meals of usually less than 10 meq/hour. there are three phases in the secretion of gastric acid which increase the secretion rate in order to digest a meal: the cephalic phase: thirty percent of the total gastric acid secretions to be produced is stimulated by anticipation of eating and the smell or taste of food. this signalling occurs from higher centres in the brain through the vagus nerve (cranial nerve x). it activates parietal cells to release acid and ecl cells to release histamine. the vagus nerve (cn x) also releases gastrin releasing peptide onto g cells. finally, it also inhibits somatostatin release from d cells. the gastric phase: about sixty percent of the total acid for a meal is secreted in this phase. acid secretion is stimulated by distension of the stomach and by amino acids present in the food. the intestinal phase: the remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distension and by amino acids. the duodenal cells release entero-oxyntin which acts on parietal cells without affecting gastrin. regulation of secretion