Sphincter

<h2 id="functioning">Functioning</h2>
<p>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.
</p><p>This is evident, for example, in the <a href="/facts/Blowhole_(biology)/5ccbhgLC">blowholes</a> of numerous <a href="/facts/Marine_mammal/WCoC2ye8">marine mammals</a>.
</p><p>Many sphincters are used every day in the normal course of <a href="/facts/Digestion/sjUBmhBS">digestion</a>. For example, the lower <a href="/facts/Oesophageal/XWa7WoWn">oesophageal</a> sphincter (or cardiac sphincter), which resides at the top of the <a href="/facts/Stomach/Q4PPDGT6">stomach</a>, is closed most of the time, keeping <a href="/facts/Stomach_acid/btVCyhYH">acids</a> and other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.
</p>
<h2 id="classifications">Classifications</h2>
<p>Sphincters can be further classified into functional and anatomical sphincters:
</p>
<ul><li>Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter.</li>
<li>Functional sphincters do not have this localised muscle thickening and achieve their sphincteric action through muscle contraction around (extrinsic) or within (intrinsic) the structure.</li></ul>
<p>Sphincters can also be voluntarily or involuntarily controlled:
</p>
<ul><li>Voluntary sphincters are supplied by <a href="/facts/Somatic_nervous_system/wEIqqhg9">somatic nerves</a>.</li>
<li>Involuntary sphincters are stimulated by <a href="/facts/Autonomic_nervous_system/hdmEY1Ay">autonomic nerves</a>.</li></ul>
<h2 id="examples">Examples</h2>

<ul><li>The <i>sphincter pupillae</i>, or <a href="/facts/Pupillary_sphincter/EKfKBoJG">pupillary sphincter</a>, belonging to the <a href="/facts/Iris_(anatomy)/e774BXL8">iris</a> in the <a href="/facts/Human_eye/a24ua4fy">eye</a>.</li>
<li>The <a href="/facts/Orbicularis_oculi_muscle/UneMBAgq">orbicularis oculi muscle</a>, a muscle around the <a href="/facts/Human_eye/a24ua4fy">eye</a>.</li>
<li>The <a href="/facts/Oesophagus/XWa7WoWn">upper oesophageal sphincters</a></li>
<li><a href="/facts/Oesophagus/XWa7WoWn">The lower esophageal sphincter, or cardiac sphincter, at the upper portion</a> (<a href="/facts/Cardia/Q4PPDGT6">cardia</a>) of the <a href="/facts/Stomach/Q4PPDGT6">stomach</a>. This sphincter prevents the <a href="/facts/Acidic/FySU18n1">acidic</a> contents of the stomach from moving upward into the <a href="/facts/Esophagus/XWa7WoWn">esophagus</a>.</li>
<li>The <a href="/facts/Pyloric_sphincter/920mWuBz">pyloric sphincter</a>, at the lower end of the stomach.</li>
<li>The <a href="/facts/Ileocecal_valve/gFp1QLwa">ileocecal sphincter</a> at the junction of the <a href="/facts/Small_intestine/1AQ8RJ4m">small intestine</a> (ileum) and the <a href="/facts/Large_intestine/vQtlNBv9">large intestine</a>, which functions to limit the reflux of colonic contents back into the ileum.</li>
<li>The <a href="/facts/Sphincter_of_Oddi/GhjmhR6v">sphincter of Oddi</a>, or Glisson's sphincter, controlling secretions from the <a href="/facts/Liver/Rtp4Fk2b">liver</a>, <a href="/facts/Pancreas/r0WPGjAq">pancreas</a> and <a href="/facts/Gall_bladder/RPdkQzQM">gall bladder</a> into the <a href="/facts/Duodenum/IAIgHBmq">duodenum</a>.</li>
<li>The <i>sphincter urethrae</i>, or <a href="/facts/Urethral_sphincter/DmnW3PoA">urethral sphincter</a>, controlling the exit of <a href="/facts/Urine/oNUYVtvX">urine</a> from the body.</li>
<li>At the <a href="/facts/Human_anus/PGkvVMYT">anus</a>, there are two anal sphincters which control the exit of <a href="/facts/Human_feces/E55Ym1Cl">feces</a> from the body, the <a href="/facts/Sphincter_ani_internus_muscle/wG6SFQvH">internal anal sphincter</a> and <a href="/facts/Sphincter_ani_externus_muscle/YIGwm0kz">external anal sphincter</a>. The inner sphincter is <a href="/facts/Smooth_muscle_tissue/mpcpTsHp">involuntary</a> and the outer is <a href="/facts/Voluntary_muscle/en1tVYFN">voluntary</a>.</li>
<li>The microscopic <a href="/facts/Precapillary_sphincter/vsTFuWMI">precapillary sphincters</a> function to control the <a href="/facts/Blood/03zEulkj">blood</a> flow into each <a href="/facts/Capillary/4BIzKJAe">capillary</a> in response to local <a href="/facts/Metabolism/WxDrm8k5">metabolic</a> activity.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a></li>
<li>The <a href="/facts/Orbicularis_oris_muscle/qxcpQe4r">orbicularis oris muscle</a>, a complex of muscles in the lips that encircles the <a href="/facts/Mouth/WthDlXsC">mouth</a>.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Vander, Arthur; Sherman, James; Luciano, Dorothy (1994). Human Physiology: The Mechanism of Body Function (Sixth Edition, International ed.). McGraw Hill, Inc. pp. 437–440. ISBN 0-07-113761-0. <a href="0-07-113761-0" target="_blank">0-07-113761-0</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Emanuel, Linda L.; Ferris, Frank D.; von Gunten, Charles F.; Hauser, Joshua M.; Von Roenn, Jamie H. (February 11, 2010). "The Last Hours of Living: Practical Advice for Clinicians". Medscape. <a href="http://www.medscape.com/viewarticle/716463_5" target="_blank">http://www.medscape.com/viewarticle/716463_5</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Vander, Arthur; Sherman, James; Luciano, Dorothy (1994). Human Physiology: The Mechanism of Body Function (Sixth Edition, International ed.). McGraw Hill, Inc. pp. 437–440. ISBN 0-07-113761-0. <a href="0-07-113761-0" target="_blank">0-07-113761-0</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
</ol>

Sphincter open-in-new

Sphincter