Open main menu

Wikipedia β

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 9000 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.[1] Sphincters relax at death, often releasing fluids.[2]

Sphincter
Anatomical terms of muscle

Contents

FunctionsEdit

Sphincters control the passage of liquids and solids. This is evident, for example, in the blowholes of numerous marine mammals.

Many sphincters are used every day in the normal course of digestion. For example, the lower esophageal sphincter (or cardiac sphincter), which resides at the top of the stomach, keeps stomach acids and other stomach contents from pushing up and into the esophagus. During contraction of sphincter/circular muscles, the lumen (opening) associated with the sphincter constricts (closes). This constriction is caused by the shortening of the sphincter muscle. Relaxation of a sphincter muscle causes it to lengthen, opening the lumen.

ClassificationsEdit

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:

ExamplesEdit

ReferencesEdit

  1. ^ a b Vander, Arthur; Sherman, James; Luciano, Dorothy (1994). Human Physiology: The Mechanism of Body Function (Sixth Edition, International Edition). McGraw Hill, Inc. pp. 437–440. ISBN 0-07-113761-0. 
  2. ^ "Medscape Log In".