Dominic Stone
A sphincter muscle is a ring-like muscle that surrounds and controls the opening and closing of bodily passages. There are over 50 types of sphincters in the human body, some of which are controlled involuntarily, some respond to stimuli, and others are controlled voluntarily. Sphincters are found in the gastrointestinal tract, urinary tract, eyes, and blood vessels. An example of a sphincter muscle is the lower oesophageal sphincter, which is closed most of the time to prevent stomach contents from moving into the oesophagus.
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What You'll Learn
Sphincter muscle pronunciation
The word "sphincter" refers to a circular muscle that usually maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals, not just humans.
There are over 50 types of sphincters in the human body, some of which are as small as a walnut, while others are microscopic. Some of the most common sphincters include:
- The anal sphincter, which regulates defecation.
- The urethral sphincter, which controls the passing of urine from the body.
- The iris sphincter, which regulates the constriction of the pupil in the eye.
- The pyloric sphincter, which is a thickening of the middle layer of stomach muscle that holds food in the stomach until it is thoroughly mixed with gastric juices.
- The lower oesophageal sphincter, which prevents the contents of the stomach from moving upward into the oesophagus.
Now, onto the pronunciation of "sphincter". The word has three syllables: "sfing", "kter", and "sphing-kter". The "ph" is silent, and the "ct" makes a "k" sound. The "s" at the beginning of the word is pronounced like a "z" sound. The "sphincter" muscle is pronounced "sfing-kter".
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Types of sphincter muscles
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 50 types of sphincters in the human body, with some sources claiming there are over 60. Some are as large as a walnut, while others are microscopic.
Involuntary and Voluntary Sphincters
Firstly, sphincters can be categorised as either involuntary or voluntary. Involuntary sphincters are stimulated by autonomic nerves, whereas voluntary sphincters are supplied by somatic nerves. An example of an involuntary sphincter is the internal anal sphincter, which is a smooth muscle regulated by the autonomic nervous system. The external anal sphincter, on the other hand, is a skeletal muscle and is under voluntary control.
Sphincters in the Gastrointestinal Tract
Many sphincters are found in the gastrointestinal (GI) tract, also known as the digestive system. The lower oesophageal sphincter, or cardiac sphincter, is located at the top of the stomach and prevents the backward flow of acidic stomach contents into the oesophagus. The pyloric sphincter is found at the lower end of the stomach and is a thickening of the middle layer of stomach muscle around the pylorus (the opening into the small intestine). This sphincter holds food in the stomach until it is thoroughly mixed with gastric juices.
Sphincters in the Urinary Tract
The urethral sphincter has inner and outer muscles that regulate urine flow through the urethra. The inner muscle is involuntary, while the outer is under voluntary control.
Sphincters in the Eyes
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction of the pupil in response to changes in light. The iris dilator muscles, in contrast, are responsible for the dilation of the pupil.
Precapillary Sphincters
Precapillary sphincters are the most common type of sphincter in the human body. They are microscopic and regulate the flow of blood into the capillaries.
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Functions of sphincter muscles
The word "sphincter" is pronounced as "sfin(g)k-ter".
Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances, such as bile, urine, and feces. They are found in the gastrointestinal (GI) tract, urinary tract, and blood vessels. There are even sphincters in the eyes, known as iris sphincters, that regulate the flow of light.
There are over 50 distinct types of sphincters in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. Some sphincters are as large as a walnut, while others are microscopic.
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 voluntary and allows the passing or holding of stool on demand. A malfunction of the anal sphincter can lead to fecal 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, while the outer muscle is voluntary. Dysfunction of the urethral sphincter can result in urinary incontinence, or the loss of bladder control.
Another example is the lower esophageal sphincter, or cardiac sphincter, which is located at the upper portion of the stomach. This sphincter remains closed most of the time, preventing stomach acids and contents from moving up into the esophagus. It opens to let swallowed food pass into the stomach.
Microscopic precapillary sphincters are the most abundant type of sphincter in the human body. These involuntary sphincters regulate blood flow into capillaries, the smallest blood vessels in the body. They ensure a consistent flow of blood and maintain pressure within the vessels, irrespective of variations in the heartbeat. If precapillary sphincters malfunction, blood can flow freely into the capillaries, creating excessive pressure and potentially leading to fluid buildup (edema) in various parts of the body.
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Locations of sphincter muscles
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 50 types of sphincters in the human body, some of which are microscopically small. Here are some of the locations of sphincter muscles in the human body:
Gastrointestinal Tract
The gastrointestinal (GI) tract contains six distinct sphincters. One example is the lower oesophageal sphincter (or cardiac sphincter), which is located at the top of the stomach and prevents the backflow of stomach contents into the oesophagus. Another example is the pyloric sphincter, situated at the lower end of the stomach.
Urinary Tract
The urethral sphincter, or sphincter urethrae, controls the exit of urine from the body. It has inner and outer muscles that regulate urine flow through the urethra.
Eyes
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction of the pupil in response to changes in light.
Blood Vessels
Precapillary sphincters are the most common type of sphincter in the human body, with millions located throughout the body. They regulate blood flow into the capillaries, ensuring consistent blood flow and pressure.
Anal Opening
The sphincter ani externus, or external anal sphincter, is one of the two sphincters at the anus that control the exit of faeces. It keeps the anal opening closed through normal contraction.
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How sphincter muscles work
A sphincter muscle is a ringlike muscle that surrounds and is able to contract or close a bodily passage or opening. Sphincters are found in many animals, and there are over 50 types in the human body, some of which are microscopic.
Each sphincter is associated with the lumen (opening) it surrounds. When 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. For example, in the blowholes of numerous marine mammals, the sphincter muscle relaxes at death, often releasing fluids and faeces.
Many sphincters are used every day in the normal course of digestion. The lower oesophageal sphincter (or cardiac sphincter), which sits at the top of the stomach, is closed most of the time, preventing stomach acids and contents from moving up into the oesophagus. However, it opens to let swallowed food pass into the stomach.
The anal sphincter, located at the opening of the rectum at the end of the digestive tract, 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, like the anal sphincter, has inner and outer muscles that regulate urine flow through the urethra. The inner muscle has involuntary control, and the outer has voluntary control. Dysfunction of the urethral sphincter can lead to urinary incontinence (loss of bladder control).
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction (narrowing) of the pupil in the eye. The iris sphincter muscles around the pupil's rim are responsible for constriction in response to changes in light (known as the pupillary light reflex). When the iris sphincter malfunctions, it can cause blurred vision and light sensitivity due to abnormally small or large pupils.
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Frequently asked questions
The 's' in sphincter is pronounced like a 'z'.
A sphincter muscle is an annular muscle that surrounds and is able to contract or close a bodily passage or opening.
Examples of a sphincter muscle include the pyloric sphincter, the cardiac sphincter, the anal sphincter, and the urethral sphincter.
When contracted, the length of the sphincter muscle is shortened, and the lumen (opening) is constricted or closed.
