chyme

Meaning: Chyme is the thick, acidic, semi-liquid mixture of partially digested food and gastric juice that forms in the stomach. Churning mixes food with HCl and pepsin, so protein digestion begins and the low pH denatures proteins. The pyloric sphincter releases chyme into the duodenum in small squirts, which gives the small intestine time to digest and absorb.

In the duodenum, acidic chyme stimulates secretin, which prompts the pancreas to release bicarbonate that neutralizes it. Fats and amino acids in chyme stimulate cholecystokinin (CCK), which triggers bile release from the gallbladder, pancreatic enzyme secretion, and slower gastric emptying.

On the MCAT: Expect questions that follow chyme through the tract and ask you to predict a change. If pancreatic bicarbonate is blocked, duodenal pH falls and pancreatic enzymes work poorly. If chyme is fat-rich, CCK rises and gastric emptying slows.

Common confusion: Chyme is not bolus or chyle. Bolus is the chewed, swallowed ball of food before it reaches the stomach; chyle is lymph rich in absorbed fats in the lacteals of the small intestine. Chyme is specifically the stomach's product, and it stays acidic until duodenal bicarbonate neutralizes it.