Sphincter
Sphincter | |
---|---|
Identifiers | |
FMA | 75004 |
Anatomical terms of muscle |
Look up sphincter in Wiktionary, the free dictionary.
A sphincter is a circular
faeces.[2]
Functioning
Each sphincter is associated with the lumen (opening) it surrounds. As long as the sphincter muscle is contracted, its length is shortened and the lumen is constricted (closed). Relaxation of the muscle causes it to lengthen, opening the lumen and allowing the passage of liquids, solids, or gases.
This is evident, for example, in the
blowholes of numerous marine mammals
.
Many sphincters are used every day in the normal course of
acids
and other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.
Classifications
Sphincters can be further classified into functional and anatomical sphincters:[citation needed]
- 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 through muscle contraction around (extrinsic) or within (intrinsic) the structure.
Sphincters can also be voluntarily or involuntarily controlled:
- Voluntary sphincters are supplied by somatic nerves.
- Involuntary sphincters are stimulated by autonomic nerves.
Examples
- The sphincter pupillae, or .
- The orbicularis oculi muscle, a muscle around the eye.
- The upper oesophageal sphincters
- acidic contents of the stomach from moving upward into the esophagus.
- 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 gall bladder into the duodenum.
- The sphincter urethrae, or urethral sphincter, controlling the exit of urinefrom the body.
- At the voluntary.
- The microscopic precapillary sphincters function to control the blood flow into each capillary in response to local metabolic activity.[1]
References
- ^ ISBN 0-07-113761-0.
- ^ Emanuel, Linda L.; Ferris, Frank D.; von Gunten, Charles F.; Hauser, Joshua M.; Von Roenn, Jamie H. (February 11, 2010). "The Last Hours of Living: Practical Advice for Clinicians". Medscape.