Sphincter

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A sphincter is a structure, usually a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. There are over 40 different sphincters in the human body; some of these sphincters are microscopic in size.

Many sphincters are used every day in the normal course of digestion. For example, the epiglottis is used to seal off the windpipe when swallowing, so as to ensure that no food or liquid enters the lungs. The use of the epiglottis is a typical example of an involuntary action by the body.

Examples of sphincters

• The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye.
• The orbicularis oris muscle, a muscle around the mouth.
• The upper esophageal sphincter
• The cardia/lower esophageal sphincter, or cardiac sphincter at the upper portion 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 sphincter, which prevents the backup of non-digested material into the small intestine.
• 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 anus, there are two sphincters which control the exit of feces from the body (see internal anal sphincter and external anal sphincter). The inner sphincter is involuntary and the outer voluntary.

Sphincters prove effective in the mediation of the entrance or release of liquids and fluids; this is evident in the blowholes of numerous marine mammals, for example.

Sphincter Video

• Video on Sphincter Control

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Acknowledgement and Attribution Regarding Sources of Content

Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .