Look up sphincter in Wiktionary, the free dictionary.
A sphincter is a circular
muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. 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.[2]
Functioning
Each sphincter is associated with the lumen (opening) it surrounds. 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.
Many sphincters are used every day in the normal course of
digestion. For example, the lower
oesophageal sphincter (or cardiac sphincter), which resides at the top of the
stomach, is closed most of the time, keeping
acids and other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.
Classifications
Sphincters can be further classified into functional and anatomical sphincters:[citation needed]
Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter.
Functional sphincters do not have this localised muscle thickening and achieve their sphincteric action through muscle contraction around (extrinsic) or within (intrinsic) the structure.
Sphincters can also be voluntarily or involuntarily controlled: