Sphincters are circular muscles that surround and control the openings of various organs in the body. They function by contracting and relaxing to regulate the passage of substances, such as food, waste, or fluids. Common examples include the esophageal sphincter, which prevents stomach acid from entering the esophagus, and the anal sphincter, which controls bowel movements. These muscles play a crucial role in maintaining bodily functions and ensuring that substances move in the right direction. Dysfunction in sphincters can lead to various health issues, such as acid reflux or incontinence, highlighting their importance in the digestive and excretory systems.