A sphincter is a circular

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

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 60 types in the human body, some microscopically small, in particular the millions of precapillary sphincters. Sphincters relax at

death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

, often releasing fluids and faeces.

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

blowhole Blowhole may refer to: * Blowhole (anatomy), the hole at the top of a whale's or other cetacean's head *Blowhole (geology), a hole at the inland end of a sea cave **Kiama Blowhole in Kiama, Australia **The Blow Hole, a marine passage between Minst ...

s of numerous

marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as Pinniped, seals, Cetacea, whales, Sirenia, manatees, sea otters and polar bears. They are an informal group, ...

s. Many sphincters are used every day in the normal course of

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

. For example, the lower oesophageal sphincter (or cardiac sphincter), which resides at the top of the stomach, is closed most of the time, keeping

acids In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...

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: * 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

pupillary sphincter The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor o ...

, belonging to the iris in the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

. * The orbicularis oculi muscle, a muscle around the

. * The upper oesophageal sphincters * The lower esophageal sphincter, or cardiac sphincter, at the upper portion ( cardia) of the stomach. This sphincter prevents the

acidic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a s ...

contents of the stomach from moving upward into the esophagus. * The pyloric sphincter, at the lower end of the stomach. *The

ileocecal sphincter The ileocecal valve (ileal papilla, ileocaecal valve, Tulp's valve, Tulpius valve, Bauhin's valve, ileocecal eminence, valve of Varolius or colic valve) is a sphincter muscle valve that separates the small intestine and the large intestine. Its c ...

at the junction of the

small intestine The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through t ...

(ileum) and the large intestine, which functions to limit the reflux of colonic contents back into the ileum. * The sphincter of Oddi, or Glisson's sphincter, controlling secretions from the

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...

pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an en ...

and gall bladder into the duodenum. * The ''sphincter urethrae'', or urethral sphincter, controlling the exit of

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellular ...

from the body. * At the anus, there are two anal sphincters which control the exit of

feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a rela ...

from the body, the internal anal sphincter and external anal sphincter. The inner sphincter is involuntary and the outer is voluntary. * The microscopic precapillary sphincters function to control the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...

flow into each capillary in response to local

metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...

activity.

References

{{Authority control Muscular system