Sphincter

A sphincter is an anatomical structure, or 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 50 in the human body, some microscopically small, in particular the precapillary sphincters.

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

Many sphincters are used everyday in the normal course of digestion and vision. 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 function of the epiglottis is a typical example of an involuntary action by the body.

Classifications
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 indirectly through muscle contraction around (extrinsic) or within (intrinsic) the structure.

Sphincters can also be voluntarily or involuntarily controlled: sphincter
 * Voluntary sphincters are supplied by somatic nerves.
 * Involuntary sphincters are stimulated by autonomic nerves.

Examples of sphincters

 * The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye.
 * The orbicularis oculi muscle, a muscle around the eye.
 * The orbicularis oris muscle, a muscle around the mouth.
 * The upper oesophageal 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 oesophagus.
 * The pyloric sphincter, at the lower end of the stomach.
 * The 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 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 is voluntary.
 * The microscopic precapillary sphincters which function to control the blood flow into each capillary in response to local metabolic activity.