Editing Sphincter

{{Short description|Circular muscle that normally maintains constriction of a natural body passage or orifice}}
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{{Wiktionary}}
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. Sphincters are found in many animals. There are over 60 types in the [[human body]],{{Citation needed|date=December 2023}} some microscopically small, in particular the millions of [[precapillary sphincter]]s.<ref name="vander-6th">{{cite book|last=Vander|first=Arthur|title=Human Physiology: The Mechanism of Body Function|url=https://archive.org/details/humanphysiology000vand|url-access=limited|year=1994|publisher=McGraw Hill, Inc|isbn=0-07-113761-0|pages=[https://archive.org/details/humanphysiology000vand/page/437 437]–440|author2=Sherman, James |author3= Luciano, Dorothy |edition=Sixth Edition, International }}</ref> Sphincters relax at [[death]], often releasing [[Body fluid|fluids]] and [[faeces]].<ref>{{cite web|url=http://www.medscape.com/viewarticle/716463_5|title=The Last Hours of Living: Practical Advice for Clinicians|website=[[Medscape]]|first1=Linda L.|last1=Emanuel|first2=Frank D.|last2=Ferris|first3=Charles F.|last3=von Gunten|first4=Joshua M.|last4=Hauser|first5=Jamie H.|last5=Von Roenn|date=February 11, 2010}}</ref>

==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.

This is evident, for example, in the [[Blowhole (biology)|blowhole]]s of numerous [[marine mammal]]s.

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 [[stomach acid|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:{{cn|date=May 2022}}
* 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:
* Voluntary sphincters are supplied by [[Somatic nervous system|somatic nerves]].
* Involuntary sphincters are stimulated by [[Autonomic nervous system|autonomic nerves]].

==Examples==
[[File:Ileocaecal sphincter.png|thumb|right|180px|Diagram of [[Ileocecal sphincter|ileocaecal valve and sphincter]]]]
* The ''sphincter pupillae'', or [[pupillary sphincter]], belonging to the [[Iris (anatomy)|iris]] in the [[human eye|eye]].
* The [[orbicularis oculi muscle]], a muscle around the [[human eye|eye]].
* The [[oesophagus#Sphincters|upper oesophageal sphincters]] 
*[[oesophagus#Sphincters|The lower esophageal sphincter, or cardiac sphincter, at the upper portion]] ([[cardia]]) of the [[stomach]]. This sphincter prevents the [[acidic]] contents of the stomach from moving upward into the [[esophagus]].
* The [[pyloric sphincter]], at the lower end of the stomach.
*The [[Ileocecal valve|ileocecal sphincter]] at the junction of the [[small intestine]] (ileum) and the [[large intestine]], which functions to limit the reflux of colonic contents back into the ileum.
* The [[sphincter of Oddi]], or Glisson's sphincter, controlling secretions from the [[liver]], [[pancreas]] and [[gall bladder]] into the [[duodenum]].
* The ''sphincter urethrae'', or [[urethral sphincter]], controlling the exit of [[urine]] from the body.
* At the [[human anus|anus]], there are two anal sphincters which control the exit of [[Human feces|feces]] from the body, the [[Sphincter ani internus muscle|internal anal sphincter]] and [[Sphincter ani externus muscle|external anal sphincter]]. The inner sphincter is [[Smooth muscle tissue|involuntary]] and the outer is [[Voluntary muscle|voluntary]].
* The microscopic [[precapillary sphincter]]s function to control the [[blood]] flow into each [[capillary]] in response to local [[Metabolism|metabolic]] activity.<ref name="vander-6th"/>
*The [[orbicularis oris muscle]], a complex of muscles in the lips that encircles the [[mouth]].

==References ==
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[[Category:Muscular system]]