Sfinkter

Sfinkter, or sphincter, refers to a circular muscle that surrounds an opening or passage in the body and constricts or relaxes to regulate flow. Sphincters may be formed by smooth muscle, skeletal muscle, or a combination, and they are controlled by autonomic (involuntary) or somatic (voluntary) nerves. They play a key role in maintaining continence and in controlling the passage of substances between regions.

In the digestive system, several major sphincters regulate movement of food and secretions. The lower esophageal sphincter (LES) sits at the junction between the esophagus and stomach and prevents reflux. The pyloric sphincter lies at the exit of the stomach, regulating gastric emptying into the small intestine. The sphincter of Oddi controls the flow of bile and pancreatic juice into the duodenum. Some structures, such as the ileocecal valve, act as barriers but are not always classified as true sphincters.

In the urinary system, the internal urethral sphincter (smooth muscle) and the external urethral sphincter (skeletal muscle) regulate the release of urine. In the anorectal region, the internal anal sphincter is involuntary, while the external anal sphincter is under voluntary control; together with pelvic floor muscles, they contribute to continence.

Other notable sphincters include the pupillary sphincter of the eye, which constricts the pupil in response to light. Disorders of sphincters can lead to incontinence, reflux, dysphagia, or obstructive symptoms depending on the site and mechanism.