Asphincteris a circularmusclethat normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in thehuman body,[citation needed]some microscopically small, in particular the millions ofprecapillary sphincters.[1]Sphincters relax atdeath,often releasingfluidsandfaeces.[2]
| Sphincter | |
|---|---|
| Identifiers | |
| TA98 | A04.0.00.028 |
| TA2 | 1992 |
| FMA | 75004 |
| Anatomical terms of muscle | |
Functioning
editEach 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 theblowholesof numerousmarine mammals.
Many sphincters are used every day in the normal course ofdigestion.For example, the loweroesophagealsphincter (or cardiac sphincter), which resides at the top of thestomach,is closed most of the time, keepingacidsand other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.
Classifications
editSphincters 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 bysomatic nerves.
- Involuntary sphincters are stimulated byautonomic nerves.
Examples
edit
- Thesphincter pupillae,orpupillary sphincter,belonging to theirisin theeye.
- Theorbicularis oculi muscle,a muscle around theeye.
- Theupper oesophageal sphincters
- The lower esophageal sphincter, or cardiac sphincter, at the upper portion(cardia) of thestomach.This sphincter prevents theacidiccontents of the stomach from moving upward into theesophagus.
- Thepyloric sphincter,at the lower end of the stomach.
- Theileocecal sphincterat the junction of thesmall intestine(ileum) and thelarge intestine,which functions to limit the reflux of colonic contents back into the ileum.
- Thesphincter of Oddi,or Glisson's sphincter, controlling secretions from theliver,pancreasandgall bladderinto theduodenum.
- Thesphincter urethrae,orurethral sphincter,controlling the exit ofurinefrom the body.
- At theanus,there are two anal sphincters which control the exit offecesfrom the body, theinternal anal sphincterandexternal anal sphincter.The inner sphincter isinvoluntaryand the outer isvoluntary.
- The microscopicprecapillary sphinctersfunction to control thebloodflow into eachcapillaryin response to localmetabolicactivity.[1]
References
edit- ^abVander, Arthur; Sherman, James; Luciano, Dorothy (1994).Human Physiology: The Mechanism of Body Function(Sixth Edition, International ed.). McGraw Hill, Inc. pp.437–440.ISBN0-07-113761-0.
- ^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.