Chyme

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Chyme is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum. It results from the mechanical and chemical breakdown of a bolus and consists of partially digested food, water, hydrochloric acid, and various digestive enzymes. Chyme slowly passes through the pyloric sphincter and into the duodenum, where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme anywhere between 40 minutes and a few hours.

With a pH of around 2, chyme emerging from the stomach is very acidic. To raise its pH, the duodenum secretes a hormone, cholecystokinin (CCK), which causes the gall bladder to contract, releasing alkaline bile into the duodenum. The duodenum also produces the hormone secretin to stimulate the pancreatic secretion of large amounts of sodium bicarbonate, which raises the chyme's pH to 7 before it reaches the ileum. As it is protected by a thick layer of mucus and utilises the neutralizing actions of the sodium bicarbonate and bile, the duodenum is not as sensitive to highly acidic chyme as the rest of the small intestine.