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 oculi muscle, a muscle around 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 is 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.

Sphincters can also 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 also can be voluntarily or involuntarily controlled:

• Voluntary sphincters are supplied by somatic nerves
• Involuntary sphincters are stimulated by autonomic nerves

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