A sphincter is a circular
muscle 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 sphincter
A precapillary sphincter is a band of contractile mural cells either classified as smooth muscle or pericytes that adjusts blood flow into capillaries. They were originally described in the mesenteric microcirculation, and were thought to only ...
s.
Sphincters relax at
death, 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
blowholes of numerous
marine mammals.
Many sphincters are used every day in the normal course of
digestion. 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 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, belonging to the
iris in the
eye.
* The
orbicularis oculi muscle, a muscle around the
eye.
* The
upper oesophageal sphincters
*
The lower esophageal sphincter, or cardiac sphincter, at the upper portion (
cardia
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is ...
) of the
stomach. This sphincter prevents the
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
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 i ...
,
pancreas 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.
Cellul ...
from the body.
* At the
anus, there are two anal sphincters which control the exit of
feces from the body, the
internal anal sphincter and
external anal sphincter
The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.
Anatomy
The external anal sphincter measures about 8 ...
. The inner sphincter is
involuntary and the outer is
voluntary.
* The microscopic
precapillary sphincter
A precapillary sphincter is a band of contractile mural cells either classified as smooth muscle or pericytes that adjusts blood flow into capillaries. They were originally described in the mesenteric microcirculation, and were thought to only ...
s 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 the cir ...
flow into each
capillary
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the bod ...
in response to local
metabolic activity.
References
{{Authority control
Muscular system