What are donut-shaped muscles that act as doorways to let materials in and out by alternately contracting relaxing?

The donut-shaped muscles that act as doorways to let materials in and out by alternately contracting and relaxing are called sphincters. Sphincters are involuntary muscles that are found in various parts of the body, including the digestive system, urinary system, and respiratory system. They function to control the flow of substances in and out of the body by opening and closing like a valve. For example, the esophageal sphincter regulates the passage of food from the esophagus into the stomach, while the anal sphincter controls the release of stool from the rectum. Sphincters play a crucial role in maintaining continence and preventing leakage of bodily fluids.