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A sphincter is a structure, usually a circular muscle
MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
, 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
Human body

The human body is the entire physical and mental structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 10 trillion Cell , the basic unit of life....
; some of these sphincters are microscopic in size.

Day-to-Day
Many sphincters are used every day in the normal course of digestion. For example, the epiglottis
Epiglottis

The epiglottis is a flap of elastic cartilage tissue covered with a mucous membrane, attached to the root of the tongue. It projects obliquely upwards behind the tongue and the hyoid bone....
 is used to seal off the windpipe when swallowing, so as to ensure that no food or liquid enters the lungs.






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Encyclopedia


A sphincter is a structure, usually a circular muscle
MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
, 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
Human body

The human body is the entire physical and mental structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 10 trillion Cell , the basic unit of life....
; some of these sphincters are microscopic in size.

Day-to-Day


Many sphincters are used every day in the normal course of digestion. For example, the epiglottis
Epiglottis

The epiglottis is a flap of elastic cartilage tissue covered with a mucous membrane, attached to the root of the tongue. It projects obliquely upwards behind the tongue and the hyoid bone....
 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
    Iris (anatomy)

    The iris is a membrane in the eye, responsible for controlling the amount of light reaching the retina. The iris consists of pigmented fibrovascular tissue known as a stroma of iris....
     in the eye
    Eye

    Eyes are Organ that detect light, and send signals along the optic nerve to the visual system and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system....
    .
  • The orbicularis oculi muscle
    Orbicularis oculi muscle

    The 'orbicularis oculi' is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short fibrous band, the medial palpebral ligament....
    , a muscle around the eye
    Eye

    Eyes are Organ that detect light, and send signals along the optic nerve to the visual system and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system....
    .
  • The orbicularis oris muscle
    Orbicularis oris muscle

    In human anatomy, the orbicularis oris muscle is the sphincter muscle around the mouth.It is also one of the muscles used in the playing of all brass instruments and some woodwind instruments....
    , a muscle around the mouth
    Mouth

    The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva....
    .
  • The upper esophageal sphincter
    Upper esophageal sphincter

    The upper esophageal sphincter refers to the superior portion of the esophagus.Unlike the lower esophageal sphincter, it consists of striated muscle and is under conscious control....
  • The cardia
    Cardia

    The cardia is the anatomy term for the junction orifice of the stomach and the esophagus. At the cardia, the mucosa of the esophagus transitions into gastric mucosa....
    /lower esophageal sphincter, or cardiac sphincter at the upper portion of the stomach
    Stomach

    In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....
    . This sphincter prevents the acidic contents of the stomach from moving upward into the esophagus
    Esophagus

    The esophagus or oesophagus , sometimes known as the gullet, is an Organ in vertebrates which consists of a Muscle tube through which food passes from the pharynx to the stomach....
    .
  • 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
    Sphincter of Oddi

    The Sphincter of Oddi is a muscular valve that controls the flow of digestive juices through the ampulla of Vater into the second part of the duodenum ....
    , or Glisson's sphincter, controlling secretions from the liver
    Liver

    The liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, a few of which are detoxification, protein synthesis, and production of biochemicals necessary for digestion....
    , pancreas
    Pancreas

    The pancreas is a gland Organ in the digestive system and endocrine system of vertebrates. It is both an endocrine gland , as well as an exocrine gland, secreting pancreatic juice containing Digestion enzymes that pass to the small intestine....
     and gall bladder into the duodenum
    Duodenum

    The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum....
    .
  • The sphincter urethrae, or urethral sphincter
    Urethral sphincter

    The urethral sphincter is a collective name for the muscles used to control the flow of urine from the urinary bladder. These muscles envelop the urethra, so that when they contract, the urethra is sealed shut....
    , controlling the exit of urine
    Urine

    Urine is a liquid waste product of the body secreted by the kidneys by a process of filtration from blood called urination and excreted through the urethra....
     from the body.
  • At the anus
    Anus

    The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as coprolite ; food material after all the nutrients have b...
    , there are two sphincters which control the exit of feces
    Feces

    Feces, faeces, or f?ces is a waste product from an animal's gastrointestinal tract expelled through the anus during defecation....
     from the body (see internal anal sphincter
    Sphincter ani internus muscle

    The Sphincter ani internus is a muscular ring which surrounds about 2.5 cm of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus....
     and external anal sphincter
    Sphincter ani externus muscle

    The Sphincter ani externus is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the integument surrounding the margin of the anus....
    ). 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 blowhole
Blowhole (biology)

In biology, a blowhole is the hole at the top of a Cetacea head through which the animal breathes air. It is Homology with the nostril of other mammals....
s 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


Trivia


In the movie Wayne's World
Wayne's World

Wayne's World was a recurring sketch from the NBC television series Saturday Night Live. It evolved from a segment titled "Wayne's Power Minute" on the CBC Television series It's Only Rock and Roll as the main character first appeared in that show....
, the character "Wayne" (played by Mike Meyers
Mike Myers (actor)

Michael John "'Mike" 'Myers is a Canada actor, comedian, screenwriter and film producer. He was a long-time cast member on the NBC sketch show Saturday Night Live in the late 1980s and the early 1990s and starred as the title characters in the films Wayne's World , Austin Powers , and Shrek...
) would say to an individual "A sphincter says what?" (asked very quickly to trick someone into saying the word "what?", thus admitting that the target is an anal sphincter).