PYLORUS

cone-shaped constriction in the gut tube of most vertebrates that demarcates the end of the stomach and the beginning of the small intestine. Its main functions are to prevent intestinal contents from reentering the stomach when the small intestine contracts and to limit the passage of large food particles or undigested material into the intestines. The internal surface is covered with a mucous-membrane lining that secretes gastric juices. Beneath the lining, circular muscle tissue allows the smaller tapered portions (sphincter) of the pylorus to open or close, permitting food to pass or be retained. The valve remains in an open or relaxed state two-thirds of the time, permitting small quantities of food to pass into the duodenum, the upper portion of the small intestine. When the duodenum begins to fill, pressure increases and causes the pyloric sphincter to contract and close. Muscular contractions (peristaltic waves) in the duodenum then push food deeper into the intestine. As the valve is relatively narrow, only small amounts of well-emulsified food can pass by it even while it is open. Surgical removal of the valve does not significantly affect the gastric emptying of the stomach. The term pylorus is used to refer to the valve opening and can also be applied to that portion of the stomach immediately above the pyloric sphincter.