Final answer:
Chyme is a thick, semi-liquid mixture formed in the stomach during mechanical digestion and protein digestion, aided by gastric juices including hydrochloric acid. It is then ushered into the small intestine for further digestion.
Step-by-step explanation:
Chyme is a thick, semi-liquid mixture that food in the gastrointestinal tract becomes by the time it leaves the stomach. During the process of mechanical digestion, the stomach muscles perform mixing waves to break down food and combine it with gastric juices, which results in the formation of chyme. This digestive mixture is highly acidic due to the presence of gastric acid, primarily hydrochloric acid, which is necessary for the enzyme pepsin to initiate protein digestion. The partially digested mixture is then carefully regulated through the pyloric sphincter into the small intestine, where it mixes with bile and continues the digestive process.