The internal surface of the pylorus is covered with a mucous-membrane lining that secretes gastric juices. Beneath the lining, circular muscle tissue allows the pyloric sphincter to open or close. This action either lets food pass into the small intestines or to be retained in the stomach. The sphincter remains relaxed, open position two-thirds of the time. This permits small amounts of food to pass into the duodenum, the upper part of the small intestine.

When the duodenum begins to fill, pressure increases and causes it to contract and close. Muscular contractions (called peristaltic waves) in the duodenum then push food deeper into the intestine. Because the pyloric sphincter is relatively narrow, only small amounts of well-emulsified food can pass through it, even when it is open. The term pylorus is used to describe or refer to the pyloric sphincter and can be used to refer to the part of the stomach immediately above the pyloric sphincter.