Sphincter

A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning.

There are over 60 types in the human body,[citation needed] some microscopically small, in particular the millions of precapillary sphincters.

[1] Sphincters relax at death, often releasing fluids and faeces.

As long as the sphincter muscle is contracted, its length is shortened and the lumen is constricted (closed).

Relaxation of the muscle causes it to lengthen, opening the lumen and allowing the passage of liquids, solids, or gases.