The anal sphincter is a group of muscles located at the end of the rectum that control the passage of stool. It consists of two main parts: the internal anal sphincter, which is involuntary and helps maintain continence, and the external anal sphincter, which is voluntary and allows for conscious control over bowel movements. These muscles work together to keep the anus closed until it is appropriate to release stool. Proper functioning of the anal sphincter is essential for maintaining bowel control and preventing issues such as fecal incontinence.