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Sphincter muscles are circular muscles that open and close bodily passages to regulate the flow of substances such as bile, urine, and faeces. There are over 50 types of sphincters in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. Sphincters are found in the gastrointestinal (GI) tract, urinary tract, and blood vessels. They can also be found in the eyes, where they regulate the flow of light. For example, the lower oesophageal sphincter, or cardiac sphincter, is closed most of the time to prevent stomach acid from moving up into the oesophagus. Sphincters can be as small as a walnut or microscopic, such as the precapillary sphincters that regulate blood flow into the capillaries.
| Characteristics | Values |
|---|---|
| Definition | Sphincters are circular or ring-like muscles that open or close passages in the body to regulate the flow of substances like urine, faeces, or bile. |
| Number of types in the human body | Over 60 |
| Microscopic sphincters | Precapillary sphincters are the most numerous type of sphincter in the human body. |
| Function | Sphincters are commonly used in the normal course of digestion. |
| Voluntary or involuntary | Sphincters can be voluntarily or involuntarily controlled. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves. |
| Sphincter muscles in the human body | Sphincter pupillae, orbicularis oculi muscle, lower oesophageal sphincter, pyloric sphincter, ileo-cecal sphincter, anal sphincter, urethral sphincter, etc. |
| Anal sphincter muscles | The internal anal sphincter and the external anal sphincter. |
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What You'll Learn
- The upper oesophageal sphincter (UES) protects the entrance to the oesophagus
- The lower oesophageal sphincter (LES) prevents stomach acid from entering the oesophagus
- The anal sphincter regulates defecation
- The urethral sphincter regulates urine flow
- The ileocecal sphincter regulates the movement of partially digested food from the small intestine to the large intestine
The upper oesophageal sphincter (UES) protects the entrance to the oesophagus
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals, and there are over 60 types in the human body. Some are microscopically small, such as the millions of precapillary sphincters that regulate blood flow into capillaries.
One of the most important human sphincter muscles is the sphincter pylori, which holds food in the stomach until it is mixed with gastric juices. Other sphincters are involved in excretion, such as the sphincter ani externus, which keeps the anal opening closed, and the sphincter urethrae, which controls urination.
The upper oesophageal sphincter (UES), also known as the pharyngoesophageal segment (PES) or inferior pharyngeal sphincter, is an involuntary sphincter found at the end of the pharynx (throat). It is a 4-cm segment of the digestive tract that separates the oesophagus from the pharynx and larynx. The UES guards the entrance of the oesophagus, controlling inflow and outflow and preventing aerophagia in the anterograde direction as well as regurgitation in the retrograde. It also prevents air from getting into the oesophagus while breathing and stops food from being breathed into the lungs. The UES is bounded anteriorly by the larynx, posterolaterally by the pharyngoesophageal muscles, superiorly by the pharynx, and inferiorly by the oesophagus. The bulk of the pressure providing the sphincteric function comes from the elastic recoil of the larynx abutting the cervical spine.
The UES is composed of the cricopharyngeus muscle (CPM), the inferior constrictor muscle, and the cranial circular esophageal muscle. The CPM is often considered synonymous with the UES, but it is just one component of a complex anatomy. The UES opens during the act of swallowing, when the CPM relaxes, the larynx elevates, and the pharynx propels the food bolus. This coordination also involves the suprahyoid muscles. The UES serves to protect the oesophagus and stomach from aerophagia and the larynx from gastric and oesophageal reflux, while facilitating normal swallow function.
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The lower oesophageal sphincter (LES) prevents stomach acid from entering the oesophagus
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopically small. Sphincters are found in many animals.
The lower oesophageal sphincter (LES) is a high-pressure zone located where the oesophagus meets the stomach. It is a ring-shaped muscle that senses when food and liquid are coming. It relaxes and lets the food pass through to the stomach. When no food or liquid is coming its way, it usually stays shut to prevent stomach acid and digestive juices from getting into the oesophagus. This is because the oesophageal mucosa cannot resist the digestive action of gastric secretions for a prolonged period. The LES is composed of intrinsic and extrinsic components. The intrinsic component of the LES consists of oesophageal muscle fibres and is under neurohormonal control. The extrinsic component consists of the diaphragmatic crura and the phrenoesophageal ligament, which provide anatomical support to the LES and further protection against gastric reflux.
Any malfunction in either of these components can lead to pathologies such as gastroesophageal reflux disease (GERD) with its associated symptoms and mucosal changes. GERD is a more severe form of acid reflux, where stomach acid persistently flows back into the oesophagus. This can cause inflammation, heartburn, a cough, chest pain, hoarseness, bad breath and/or trouble swallowing. Over time, GERD can cause significant damage to the oesophagus.
To treat GERD, antacids can be used to neutralise stomach acids. Acid blockers can also be used to decrease acid production. Baclofen is used to reduce the relaxation of the LES so that it doesn't open when it's not supposed to. Surgery is an option for patients who do not respond to conservative management. The goal of surgery is to reinforce the LES, close the crura of the diaphragm, and wrap the upper part of the stomach around the end of the oesophagus so that acids from the stomach cannot back up into the oesophagus.
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The anal sphincter regulates defecation
Sphincters are circular muscles that maintain constriction of a natural body passage or orifice and relax as required by normal physiological functioning. There are over 50 types of sphincters in the human body, with some sources stating there are over 60. They are found in many animals.
The anal sphincter is located at the opening of the rectum at the end of the digestive tract. It has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out. The external anal sphincter is a skeletal muscle and acts voluntarily, responding to commands to contract and maintain continence when faeces are propelled into the rectum. It maintains its tone to keep the orifice of the anal canal closed and relaxes upon defecation through the defecation reflex. The internal anal sphincter is a smooth muscle, measuring about 30mm in height and 3mm in thickness. It originates from extensions of the circular layers of the rectum into the anal canal. The autonomic nervous system regulates its activity, which is not affected by human volition, thus making it involuntary in action.
A malfunction of the anal sphincter can cause faecal incontinence, or the loss of bowel control. This can cause poop to come out when you don't want it to.
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The urethral sphincter regulates urine flow
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopically small. Sphincters are found in many animals.
The urethral sphincter is a primary muscle for prohibiting urination. It has two muscles: the internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter is located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder. The external urethral sphincter originates at the ischiopubic ramus and inserts into the intermeshing muscle fibres from the other side. Both muscles work together with the bladder to get urine out of the body. When either of these muscles contracts, the urethra is sealed shut, prohibiting urination.
The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra. It is made of a layer of smooth muscle surrounded by striated muscle and is under involuntary or autonomic control. The external urethral sphincter, on the other hand, controls voluntary urine flow from the bladder to the urethra. Unlike the internal sphincter muscle, the external sphincter is made of skeletal muscle and is under voluntary control by the somatic nervous system.
In males, the internal urethral sphincter has an additional function of preventing the flow of semen into the male bladder during ejaculation. Females have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. Any damage to the urethral sphincter muscles can lead to urinary incontinence, the loss of bladder control.
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The ileocecal sphincter regulates the movement of partially digested food from the small intestine to the large intestine
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals, and there are over 50 types in the human body. Some are as small as a walnut, while others are microscopic. They open and close passages in the body to regulate the flow of substances, such as bile, urine, and faeces.
The ileocecal sphincter is an involuntary sphincter located at the junction of the small intestine (ileum) and the large intestine (colon). It is also known as the ileocecal valve. The ileocecal sphincter functions to limit the reflux of colonic contents back into the ileum. It is thought to regulate the movement of partially digested food from the ileum (the last section of the small intestine) to the large intestine. This ensures that the body has sufficient time to absorb nutrients before waste enters the colon.
The ileocecal sphincter is one of six distinct sphincters situated within the GI tract (also known as the digestive system). Some sphincters consist of smooth muscles that function involuntarily or by reflex, while others are made of striated muscles that function voluntarily. The upper oesophageal sphincter (UES), for example, is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the oesophagus, which carries food from the throat to the stomach. The swallowing reflex triggers the opening of this sphincter. The UES prevents air from getting into the oesophagus while breathing and plays a role in burping and vomiting.
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 contents from flowing back into the oesophagus. When functioning properly, the LES relaxes to allow food to enter the stomach and then contracts to prevent acid reflux. The pyloric sphincter, at the lower end of the stomach, regulates the passage of partially digested food, known as chyme, from the stomach into the small intestine. It ensures that the stomach contents are properly mixed with gastric juices and only released in small amounts.
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Frequently asked questions
A sphincter muscle is a circular or ring-like muscle that opens or closes passages in the body to regulate the flow of substances like urine, faeces, bile, food, liquids, solids, or gases.
There are over 60 types of sphincter muscles in the human body. They are found in the gastrointestinal (GI) tract, the digestive system, the eye, and the urethra.
Sphincter muscles can be voluntarily or involuntarily controlled. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves. Examples of voluntary sphincters include the external anal sphincter and the urethral sphincter. Examples of involuntary sphincters include the internal anal sphincter, the ileocecal sphincter, and the upper oesophageal sphincter (UES).
Malfunctioning of sphincter muscles can lead to various problems depending on their location. For example, UES dysfunction can cause difficulty swallowing, acid reflux, and gastroesophageal reflux disease (GERD). Sphincter of Oddi (SO) dysfunction can cause bile and pancreatic enzymes to back up into the stomach, leading to severe stomach and chest pain.
