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Sphincter
A sphincter is a cylindrical 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, some microscopically small, in particular the millions of precapillary sphincters. Sphincters relax at death, possibly releasing fluids.〔http://www.medscape.com/viewarticle/716463_5〕
==Functions==
Sphincters prove effective in the mediation of the entrance or release 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 and vision. For example, the lower esophageal sphincter (aka: 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 associated with the sphincter constricts (closes). This constriction is caused from the "shortening" of the sphincter muscle. While relaxation of a sphincter muscle causes the lumen to open, which is caused by the lengthening of the associated sphincter muscle.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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