The pyloric sphincter is a ring of smooth muscle at the stomach’s outlet that guards the passage into the duodenum, the first part of the small intestine. It opens briefly and rhythmically to squirt small jets of chyme, the semi-liquid mass of partly digested food, into the intestine, then closes to prevent backflow. This metering action is coordinated with stomach contractions and hormonal signals: acid and fat in the duodenum trigger reflexes that slow emptying, giving enzymes and bile time to work. When the sphincter is abnormally thickened, as in infantile pyloric stenosis, food cannot pass and projectile vomiting results, requiring surgery. Its precise control of gastric emptying strongly influences satiety, blood sugar responses, and drug absorption.