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A sphincter muscle is a ring-like muscle that surrounds and regulates the opening and closing of bodily passages or orifices. There are over 50 types of sphincters in the human body, some of which are microscopic in size. They are found in the gastrointestinal (GI) tract, urinary tract, and blood vessels. Sphincters can be voluntarily or involuntarily controlled. For example, the internal anal sphincter is involuntary and prevents stool from leaking out, while the external anal sphincter is voluntary and allows for the passing of stool.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles that surround and are able to contract or close a bodily passage or opening |
| Number of types in the human body | Over 50 distinct types, some sources say over 60 |
| Size | Some are as large as a walnut, while others are microscopic |
| Function | Some function involuntarily, some respond to stimuli, and others are controlled voluntarily |
| Types | Anal sphincter, urethral sphincter, iris sphincter, precapillary sphincter, pyloric sphincter, ileocecal sphincter, sphincter of Oddi, orbicularis oris muscle |
| Anal sphincter characteristics | Consists of inner and outer muscles; the inner sphincter is involuntary, and the outer is voluntary |
| Urethral sphincter characteristics | Controls the passing of urine from the body; has inner and outer muscles that regulate urine flow |
| Iris sphincter characteristics | Involuntary sphincter that regulates the constriction of the pupil in the eye |
| Precapillary sphincter characteristics | Control the flow of blood through smaller vessels |
| Pyloric sphincter characteristics | Located at the lower end of the stomach |
| Ileocecal sphincter characteristics | Located at the junction of the small intestine and the large intestine, and functions to limit the reflux of colonic contents back into the ileum |
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What You'll Learn
- Sphincter muscles are ring-like muscles that open and close bodily passages
- There are over 50 types of sphincters in the human body, some are microscopic
- Sphincters can be voluntarily or involuntarily controlled
- Anal sphincters control the passing of stool from the body
- Sphincters can malfunction, causing health issues like acid reflux and incontinence
Sphincter muscles are ring-like muscles that open and close bodily passages
The gastrointestinal (GI) tract contains six distinct sphincters, some of which are smooth muscles that function involuntarily or by reflex, while others are made of striated muscles that function voluntarily. The lower oesophageal sphincter, or cardiac sphincter, is closed most of the time, preventing the acidic contents of the stomach from moving upward into the oesophagus. The pyloric sphincter, located at the lower end of the stomach, is another important sphincter.
The anal sphincter, which regulates defecation, has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is predominantly voluntary and allows the passing or holding of stool on demand. The urethral sphincter, also known as the sphincter urethrae, controls the passing of urine from the body. Like the anal sphincter, it has inner and outer muscles that regulate urine flow through the urethra.
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction of the pupil in the eye. The iris sphincter muscles around the pupil's rim are responsible for constriction, while the iris dilator muscles radiating outward are tasked with dilation.
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There are over 50 types of sphincters in the human body, some are microscopic
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, with some sources claiming over 60. They range in size from as large as a walnut to being microscopic.
Sphincters open and close passages in the body to regulate the flow of substances such as bile, urine, and faeces. They are found in the gastrointestinal (GI) tract, the urinary tract, and blood vessels. There are even sphincters in the eyes, which regulate the flow of light.
Some sphincters function involuntarily, some respond to stimuli, and others are controlled voluntarily. Anatomical sphincters have a localized and often circular muscle thickening to facilitate their action as a sphincter. Functional sphincters do not have this localized muscle thickening and achieve their sphincteric action through muscle contraction around or within the structure.
The anal sphincter, for example, 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, while the outer sphincter is predominantly voluntary and allows the passing or holding of stool on demand. A malfunction of the anal sphincter can cause faecal incontinence, or the loss of bowel control.
The urethral sphincter, also known as the sphincter urethrae, controls the passing of urine from the body. Like the anal sphincter, it has inner and outer muscles that regulate urine flow through the urethra. The inner muscle is involuntary, and the outer muscle is voluntary.
The microscopic precapillary sphincters are the most numerous type of sphincter in the human body. These involuntary sphincters regulate the flow of blood into the tiniest blood vessels, called capillaries. There are millions of these microscopic sphincters located throughout the body, including those that service capillaries in the brain. They are important for maintaining consistent blood flow and pressure within the vessels, irrespective of natural variations in a heartbeat.
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Sphincters can be voluntarily or involuntarily controlled
A sphincter is a circular muscle that opens and closes passages in the body to regulate the flow of substances. 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.
The upper esophageal sphincter is the only sphincter that is under voluntary control. It opens when we consciously swallow to allow food to pass from the pharynx into the esophagus. The other sphincters, such as the lower esophageal sphincter, operate involuntarily. The lower esophageal sphincter functions automatically to prevent reflux of stomach contents back into the esophagus.
The anal sphincter has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is predominantly voluntary and allows one to pass or hold stool on demand. Malfunctions of the anal sphincter can cause fecal incontinence.
The urethral sphincter also has inner and outer muscles that regulate urine flow through the urethra. The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, while the external urethral sphincter controls voluntary urine flow. Damage to these muscles can lead to urinary incontinence.
The iris sphincter, also known as the pupillary sphincter, is an involuntary sphincter that regulates the constriction of the pupil in the eye. The iris sphincter muscles around the pupil's rim are responsible for constriction, while the iris dilator muscles radiating outward from the pupils are tasked with dilation.
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Anal sphincters control the passing of stool from the body
A sphincter muscle is any of the ring-like muscles that surround and are able to contract or close a bodily passage or opening. There are over 50 types of sphincters in the human body, some functioning involuntarily, some responding to stimuli, and others controlled voluntarily. Sphincters are found in the gastrointestinal (GI) tract, the urinary tract, and even the eyes, where they regulate the flow of light.
The anal sphincter is a group of muscles at the end of the rectum, which is located at the last few inches of the large intestine (colon). The anal sphincter surrounds the anus and controls the release of stool, thereby maintaining continence. The rectum is the section of the large intestine that comes just before the anus, and it is where poop collects before it is ready to come out. When the rectum is full, nerves trigger the urge to have a bowel movement, and the internal anal sphincter relaxes automatically.
The anal sphincter has two muscles: the internal anal sphincter and the external anal sphincter. The internal anal sphincter is an involuntary smooth muscle, like the muscles of the intestines, and it contracts during rest and sleep, preventing stool, small amounts of liquid, and gas from escaping unexpectedly. The external anal sphincter is a voluntary muscle that surrounds the internal anal sphincter. It is the muscle that is squeezed when one feels the urge to go to the bathroom but is not near one. These two muscles work together to produce a bowel movement.
If the anal sphincter muscles are damaged or malfunction, fecal incontinence, or the loss of bowel control, can occur. This can be caused by a flare-up of inflammatory diseases, such as IBD, or by a problem with the way the muscles contract and relax during defecation, known as anismus. Anal stenosis, a condition where the body canal narrows, usually due to scar tissue, can also affect the anal sphincter.
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Sphincters can malfunction, causing health issues like acid reflux and incontinence
A sphincter is a circular muscle that opens and closes passages in the body 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 can malfunction, causing various health issues. For example, the lower oesophageal sphincter (LES) is a valve at the bottom of the oesophagus that usually keeps stomach contents from rising back up. Acid reflux occurs when the LES weakens or relaxes enough to let acid pass. Acid reflux can progress into gastroesophageal reflux disease (GERD) if it occurs more than twice a week. GERD can damage oesophageal tissues over time.
Temporary factors such as a large meal, lying down, or pregnancy can cause the LES to relax. Additionally, pregnancy hormones like relaxin, estrogen, and progesterone can worsen reflux symptoms. In some cases, a hiatal hernia may cause GERD by pushing the upper part of the stomach and LES above the diaphragm, a muscle that separates the stomach from the chest.
Another example of a sphincter malfunction is sphincter of Oddi dysfunction, where the sphincter fails to open, causing a backup of digestive juices and severe abdominal pain. This condition can develop after a cholecystectomy, the procedure to remove the gallbladder.
Furthermore, the anal sphincter regulates defecation. A malfunction can lead to faecal incontinence, or the loss of bowel control. Similarly, the urethral sphincter controls the passing of urine, and its malfunction can cause urinary incontinence.
In summary, sphincter malfunctions can lead to health issues such as acid reflux, GERD, faecal and urinary incontinence, and sphincter of Oddi dysfunction. Treatment options vary depending on the type of sphincter involved and the severity of the condition.
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Frequently asked questions
A sphincter muscle is a circular muscle that surrounds a bodily passage or opening. It maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning.
There are over 50 types of sphincter muscles in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. Some examples include the anal sphincter, urethral sphincter, iris sphincter, and precapillary sphincter.
The anal sphincter is located at the opening of the rectum and regulates defecation. It has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is voluntary and allows you to pass or hold stool on demand.
The urethral sphincter controls the passing of urine from the body. Like the anal sphincter, it also has inner and outer muscles that regulate urine flow.
The iris sphincter, also known as the pupillary sphincter, is an involuntary muscle that regulates the constriction of the pupil in response to changes in light.
