Look up sphincter in Wiktionary, the free dictionary.
A sphincter is an anatomical structure, 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 50 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.
Sphincters prove effective in the mediation of the entrance or release of liquids and fluids; this is evident, for example, in the blowholes of numerous marine mammals.
Many sphincters are used every day in the normal course of digestion and vision. For example, the cardiac sphincter resides at the top of the stomach, to keep stomach acids and other stomach contents from pushing up and into the esophagus.
Sphincters can be further classified into functional and anatomical sphincters:
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:
^ abVander, Arthur; Sherman, James; & Luciano, Dorothy (1994). Human Physiology: The Mechanism of Body Function (Sixth Edition, International Edition). McGraw Hill, Inc. pp. 437–440. ISBN0-07-113761-0.