Sphincter

From Wikipedia, the free encyclopedia

A sphincter is a circular muscle which normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. There are at least 42 different sphincters in the human body, some of them microscopic in size.

Some examples of sphincters include:

• The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye.
• The orbicularis oris muscle, a muscle around the mouth.
• 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 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.