Chyme

Chyme is the semi-fluid, partially digested content that leaves the stomach and enters the small intestine, mainly the duodenum. It results from mechanical mixing of food by stomach muscles and chemical digestion by gastric secretions, including hydrochloric acid and enzymes such as pepsin. Chyme is not a distinct substance but a mixture of digested food and gastric juice.

The acidity and consistency of chyme change as it moves through the stomach and into the intestine. In the stomach, chyme is highly acidic, with a pH around 1–2. As it enters the duodenum, bicarbonate-rich pancreatic secretions and bile begin to neutralize the acid, gradually increasing the pH. The content also becomes more liquid as digestion proceeds. Gastric emptying is influenced by several factors, including volume, osmolarity, and the presence of fat or solids, with fats and large solids typically slowing emptying more than liquids.

Regulation and transit into the small intestine involve the pyloric sphincter and coordinated neural and hormonal signals. When chyme reaches the duodenum, cells release secretin and cholecystokinin (CCK), which slow gastric emptying and stimulate pancreatic bicarbonate and enzyme secretion, as well as bile release. This regulatory loop helps balance digestion with the intestine’s capacity to process and absorb nutrients.

In the duodenum, chyme mixes with pancreatic juice and bile, enabling continued digestion of carbohydrates, proteins, and fats. The neutralization of acid, enzymatic activity, and fat emulsification facilitate nutrient absorption and influence subsequent intestinal motility.