The Digestive System and the Liver - Digestive sphincters
If you think about it, the gastrointestinal tract has to be equipped with a number of gates that can open or shut, depending on the amount of food that is passing through. Otherwise, the food might push through so fast that little nourishment could be extracted from it: we would feel hungry one minute and glutted the next. The gastrointestinal tract is thus equipped at critical junctures with a number of muscular valves, or sphincters , which, usually under the direction of the autonomic nervous system, can regulate the movement of food through the digestive tube. Another function of a sphincter is to prevent backflow of partially digested food.
The muscles of a sphincter are often described as “pursestring muscles” because the way they draw together the sides of the digestive tube is roughly similar to drawing up the strings of a purse. The first of these pursestring valves occurs at the cardia , the opening where the esophagus meets the stomach, and is called the cardiac sphincter , from its location almost directly in front of the heart. (But there is no physical connection.) Another important muscle ring is the pyloric sphincter , at the opening called the pylorus , located at the other end of the stomach, at the connection between stomach and small intestine. The release of waste from the rectum is controlled, partly voluntarily, by an anal sphincter , located at the anus , which marks the end of the tract.