A sphincter is a circular band of muscle that surrounds and controls the opening of a passage in the body. These muscles can tighten or relax to regulate the flow of substances, such as food or waste. For example, the esophageal sphincter helps keep food in the stomach, while the anal sphincter controls bowel movements. Sphincters are essential for various bodily functions, ensuring that processes like digestion and excretion happen smoothly. They work automatically, but we can also control some sphincters consciously, like when we decide to hold in urine using the urinary sphincter.