Gut

Stomach acid is neutralized in the gut - (Pancreas, Bile, Bacteria)

Claude AI May 2026

The acidity drops sharply once chyme (the partially digested stomach contents) passes through the pyloric sphincter into the duodenum, the first part of the small intestine. Stomach pH runs around 1.5–3.5; by the time material is well into the small intestine, pH is roughly 6–7.4.

The neutralization happens through a few coordinated mechanisms:

The main one is pancreatic bicarbonate. When acidic chyme hits the duodenum, S cells in the duodenal lining detect the low pH and release the hormone secretin into the bloodstream. Secretin signals the pancreas to pump out a bicarbonate-rich fluid (sodium bicarbonate) through the pancreatic duct into the duodenum. That bicarbonate reacts with HCl to produce salt, water, and CO₂, raising the pH.

Bile contributes too. Bile from the liver (stored in the gallbladder) is mildly alkaline and enters the duodenum through the common bile duct, adding to the buffering, though its main job is emulsifying fats.

Brunner's glands, which sit in the submucosa of the duodenum, secrete an alkaline mucus that protects the intestinal lining from any residual acid before pancreatic juice fully neutralizes things.

This neutralization isn't just incidental — it's essential. Pancreatic digestive enzymes (trypsin, lipase, amylase) work optimally near neutral pH and would be inactivated by stomach-level acidity. The duodenum is also far more vulnerable to acid damage than the stomach, which has a thick protective mucus layer the small intestine lacks.

Further downstream, the colon trends slightly acidic again (around pH 5.5–7) because gut bacteria ferment fiber into short-chain fatty acids — a different mechanism entirely from gastric acid.

See also web

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