Sphincter refers to a kind of annular muscle distributed on the wall of some lumens in humans and animals, which is often particularly thickened. Sphincters in human body exist in digestive tract and urinary system.

Sphincter can close the lumen when it contracts, and open the lumen when it relaxes, and it is usually in a contracted state. Generally controlled by autonomic nerves or hormones.

The contraction and relaxation of sphincter plays an important role in maintaining normal physiological function. Mainly includes: pyloric sphincter, which can prevent duodenal contents from flowing into the stomach. When the bladder sphincter contracts, the internal urethral orifice is closed to prevent urine leakage. If the function is impaired, it will lead to urinary incontinence. Anal sphincter plays an extremely important role in controlling fecal excretion. If injured, it will lead to fecal incontinence.