Final answer:
The pyloric sphincter acts as the gatekeeper between the stomach and the duodenum, controlling the passage of chyme. It is modulated by various digestive reflexes to optimize digestion.
Step-by-step explanation:
The gatekeeper that regulates the passage of chyme from the stomach to the duodenum is the pyloric sphincter. This muscular valve controls the flow of partially digested food, or chyme, ensuring that only small amounts are released into the duodenum at a time. During the intestinal phase of gastric secretion, the enterogastric reflex plays a crucial role by instructing the pyloric sphincter to close when the duodenum is overwhelmed with chyme, thus preventing more chyme from entering and allowing sufficient time for digestion and absorption. In contrast, the hepatopancreatic sphincter (or sphincter of Oddi) regulates the flow of bile and pancreatic juice into the duodenum, not chyme.