A sphincter is an anatomical

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 structure, or a circular muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals; there are over 50 in the human body

Human body

The human body is the entire 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 100 trillion cells, the basic unit of life...

, some microscopically small, in particular the precapillary sphincter

Precapillary sphincter

The precapillary sphincter is a band of smooth muscle that adjusts the blood flow into each capillary. At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and...

s.

Functions

Sphincters prove effective in the mediation of the entrance or release of liquids and fluids; this is evident, for example, in the blowhole

Blowhole (biology)

In biology, a blowhole is the hole at the top of a cetacean's head through which the animal breathes air. It is homologous with the nostril of other mammals. As whales reach the water surface to breathe, they will forcefully expel air through the blowhole. Not only is air expelled, but mucus and...

s of numerous marine mammals.

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

Epiglottis

The epiglottis is a flap that is made of elastic cartilage tissue covered with a mucous membrane, attached to the entrance of the larynx. It projects obliquely upwards behind the tongue and the hyoid bone, pointing dorsally. The term, like tonsils, is often incorrectly used to refer to the uvula...

 is used to seal off the windpipe

Vertebrate trachea

In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...

 when swallowing, so as to ensure that no food or liquid enters the lungs. The function of the epiglottis is a typical example of an involuntary action by the body.

Classifications

Sphincters can 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 can also be voluntarily or involuntarily controlled:

• Voluntary sphincters are supplied by somatic nerves
  Somatic nervous system
  The somatic nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles...
  
  .
• Involuntary sphincters are stimulated by autonomic nerves
  Autonomic nervous system
  The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
  
  .

Examples

• The sphincter pupillae, or pupillary sphincter, belonging to the iris
  Iris (anatomy)
  The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
  
   in the eye
  Human eye
  The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
  
  .
• 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.From...
  
  , a muscle around the eye
  Human eye
  The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
  
  .
• The orbicularis oris muscle
  Orbicularis oris muscle
  In human anatomy, the orbicularis oris muscle is the sphincter muscle around the mouth."Orbicularis Oris is a complex of muscles in the lips that encircle the mouth; until recently it was misinterpreted as a sphincter, or circular muscle, but it is actually composed of four independent quadrants...
  
  , a muscle around the mouth
  Mouth
  The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
  
  .
• The upper oesophageal sphincter.
• The cardia
  Cardia
  The cardia is the anatomical term for the part of the stomach attached to the esophagus. The cardia begins immediately distal to the z-line of the gastroesophageal junction, where the squamous epithelium of the esophagus gives way to the columnar epithelium of the gastrointestinal tract.Just...
  
   (lower esophageal sphincter), or cardiac sphincter at the upper portion of the stomach
  Stomach
  The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
  
  . This sphincter prevents the acidic contents of the stomach from moving upward into the oesophagus.
• The pyloric sphincter, at the lower end of the stomach.
• The ileocecal sphincter
  Ileocecal valve
  The ileocecal valve, or ileocaecal valve, is of a bilabial papilla structure with physiological sphincter muscle situated at the junction of the small intestine and the large intestine, with recent evidence indicating an anatomical sphincter may also be present in humans) Its critical function is...
  
   at the junction of the small intestine
  Small intestine
  The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
  
   (ileum) and the large intestine
  Large intestine
  The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
  
  , which functions to limit the reflux of colonic contents back into the ileum.
• The sphincter of Oddi
  Sphincter of Oddi
  The sphincter of ampulla or 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. It is named after Ruggero Oddi...
  
  , 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, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
  
  , pancreas
  Pancreas
  The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
  
   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 term urethral sphincter refers to one of two muscles used to control the exit of urine in the urinary bladder through the urethra. The two muscles are the external urethral sphincter and the internal urethral sphincter...
  
  , controlling the exit of urine
  Urine
  Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
  
   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 control the expulsion of 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,...
  
  , there are two sphincters which control the exit of feces
  Feces
  Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
  
   from the body (see internal anal sphincter
  Sphincter ani internus muscle
  The Sphincter ani internus is a muscular ring that 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.-Anatomy:...
  
  ). The inner sphincter is involuntary and the outer is voluntary.
• The microscopic precapillary sphincter
  Precapillary sphincter
  The precapillary sphincter is a band of smooth muscle that adjusts the blood flow into each capillary. At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary. This is called the precapillary sphincter. This sphincter can open and...
  
  s which function to control the blood
  Blood
  Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
  
   flow into each capillary
  Capillary
  Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
  
   in response to local metabolic
  Metabolism
  Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
  
  activity.