A ring-shaped muscle that relaxes or tightens to open or close a passage or opening in the body. Examples are the anal sphincter (around the opening of the anus) and the pyloric sphincter (at the lower opening of the stomach).

What does the sphincters do in the digestive system?

What's its function? The pyloric sphincter serves as a kind of gateway between the stomach and the small intestine. It allows the contents of the stomach to pass into the small intestine. It also prevents partially digested food and digestive juices from reentering the stomach.

What does the sphincter prevent?

With the exception of the internal anal sphincter, sphincters function to prevent the backward movement of intraluminal contents. The internal anal sphincter prevents uncontrolled movement of intraluminal contents through the anus. The lower esophageal sphincter prevents reflux of gastric acid into the esophagus.