![]() This pH profile can be changed by surgical interventions and in inflammatory bowel disease ( 8, 28). While HCl and bicarbonate (HCO 3 −) secretion are the major determinants of luminal pH in the foregut, luminal pH in the colon depends on mucosal HCO 3 − and lactate production as well as on microbial transformation of carbohydrates to short chain fatty acids and formation of ammonia ( 28). The pH profile in the GI lumen of healthy subjects shows a distinct shape ( 8, 28), with peaks of acidity in the stomach and proximal large bowel ( Table 1). Importantly, acid sensors are not only relevant to control the secretion and actions of gastric acid but also to detect tissue acidosis resulting, e.g., from ischemia, inflammation, microbial activity, malignant tumor growth and gastrointestinal (GI) motor stasis ( 8, 28). If the pathophysiologic impact of gastric acid gets out of control, acid-related diseases including gastritis, gastroduodenal ulceration, dyspepsia and gastroesophageal reflux disease (GERD) may ensue. Both strategies require an acid surveillance system among which acid-sensitive afferent neurons play a particular role ( 10, 11). The autoaggressive potential of HCl is kept in check by an elaborate network of mucosal defense mechanisms and the functional compartmentalization of the esophago-gastro-duodenal region ( Figure 1). Most tissues would rapidly disintegrate if exposed to this concentration of HCl, yet gastric acid is essential for the digestive breakdown of food and elimination of ingested pathogens. Thus, the average diurnal pH in the empty human stomach is around 1.5 ( 25). ![]() It is one of the most remarkable physiologic achievements that the parietal cells can secrete hydrochloric acid (HCl) to yield a proton (H +) concentration in the gastric lumen that is more than 6 orders of magnitude higher than in the interstitial space of the gastric lamina propria. Acidity in the stomach and adjacent gut regionsĮver since the landmark paper “On the nature of acid and saline matters usually existing in the stomach of animals” was presented by William Prout to the Royal Society of London on December 11, 1823, has the stomach been known as a highly productive source of acid in the foregut.
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