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Sphincters are ring-shaped muscles that open and close to regulate the flow of substances, such as food, liquid, and
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What You'll Learn
Involuntary sphincters
The human body has over 60 types of sphincters, some of which are involuntary. One example is the internal anal sphincter, which is one of two sphincters at the anus that control the exit of faeces from the body. The inner sphincter is involuntary, while the outer (or external anal sphincter) is voluntary.
Another example is the internal urethral sphincter, which is composed of smooth muscle and regulates the involuntary control of urinary flow from the bladder to the urethra. It works in tandem with the detrusor muscle of the bladder. In men, the internal urethral sphincter also functions to prevent the backflow of semen into the bladder during ejaculation.
The ileocecal sphincter is another involuntary sphincter. It controls the movement of material from the ileum into the large intestine and prevents the backward movement of material from the colon into the ileum.
There are also millions of precapillary sphincters in the human body, which are the most numerous type of sphincter. These involuntary sphincters regulate the flow of blood into capillaries in response to local metabolic activity.
The pupillary sphincters, or sphincter pupillae, are a pair of involuntary sphincters that are involved with pupil dilation in response to light.
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Sphincters in the digestive tract
Sphincters are ring-shaped muscles that open and close to regulate the flow of substances, including food, liquids, and waste, through bodily passages. They are found in the digestive tract, urinary tract, blood vessels, and eyes. There are over 60 types of sphincters in the human body, and they are either under voluntary or involuntary control.
The function of sphincters in the digestive tract is to act as one-way valves by contracting and closing to regulate and coordinate the flow of gastrointestinal contents. They include both skeletal and smooth muscles located in specific sites throughout the gut. Smooth muscle sphincters, such as the lower esophageal sphincter, pyloric sphincter, sphincter of Oddi, ileocecal sphincter, and internal anal sphincter, are autonomic organs controlled by complex interactions between extrinsic nerves from the central nervous system and intrinsic control by the enteric nervous system. Skeletal muscle sphincters, such as the upper esophageal sphincter and external anal sphincter, are under voluntary control.
The lower esophageal sphincter (LES) is located between the esophagus and stomach. When this sphincter malfunctions and does not close completely, it can lead to gastroesophageal reflux disease (GERD), allowing stomach acid to backflow into the esophagus and causing heartburn and other symptoms. The pyloric sphincter, also known as sphincter pylori, is a thickening of the middle layer of stomach muscle around the pylorus, which opens into the small intestine. It holds food in the stomach until it is thoroughly mixed with gastric juices. Dysfunction of the pyloric sphincter can lead to biliary reflux, where bile backs up into the stomach. The ileocecal sphincter (ICS) is located between the small intestine and cecum. The internal anal sphincter (IAS) is one of two sphincters that control the anal opening, along with the external anal sphincter, which is under voluntary control.
The sphincter of Oddi is a fibromuscular sheath that encircles the terminal portion of the common bile duct, main pancreatic duct, and common channel in the second portion of the duodenum. It contracts phasically during the digestive period to promote the flow of bile and pancreatic secretions into the duodenum. Its activity is regulated by neural and hormonal factors, with endogenous substances such as motilin, cholecystokinin (CCK), vasoactive intestinal peptide (VIP), and nitric oxide influencing its contractions and relaxations.
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Sphincters in the urinary tract
Sphincters are ring-shaped muscles that open and close to regulate the flow of substances through passages in the body. There are over 60 types of sphincters in the human body, with some being under voluntary control and others under involuntary control.
The urinary tract is one of the bodily systems that contain sphincters. The urethral sphincter is a muscular structure that regulates the outflow of urine from the bladder into the urethra. There are two urethral sphincters, the external and internal sphincters. The internal urethral sphincter is located at the junction of the neck of the bladder and urethra and controls involuntary urine flow from the bladder to the urethra. It is an involuntary muscle that works in tandem with the detrusor muscle of the bladder. The external urethral sphincter, on the other hand, is a voluntary muscle that controls voluntary urine flow from the bladder to the urethra. It is located on the pelvic floor and helps to "hold it in" when the bladder is full.
In males, the internal urethral sphincter has an additional function of preventing the flow of semen into the bladder during ejaculation. Females have a more complex external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibres wrap around the vagina and urethra, and contraction leads to the constriction of both.
Damage to the urethral sphincter or surrounding muscles can lead to urinary incontinence, the loss of bladder control. This can range from releasing a small amount of urine when laughing or sneezing to having a sudden and strong urge to urinate. Stress urinary incontinence is a common problem, accounting for up to 88% of all incontinent patients. It occurs when there is a problem with the closing mechanism at the urinary tract outlet due to issues such as direct trauma, muscle tone, or damage to surrounding tissues. Childbirth, obesity, and age can be risk factors for stress incontinence in women, while prostate surgery and radiation therapy can cause it in men. Treatment options for urinary incontinence include lifestyle changes, medication, pelvic floor exercises, and in some cases, surgery.
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Sphincters in blood vessels
Sphincters are ring-shaped muscles that open and close to regulate the flow of substances through bodily passages. There are over 60 types of sphincters in the human body, including in the digestive tract, urinary tract, blood vessels, and eyes. Some sphincters are under voluntary control, while others are involuntary.
Precapillary sphincters are the most common sphincters in the human body. They are involuntary sphincters that regulate the flow of blood into capillaries, the smallest blood vessels. Precapillary sphincters are bands of contractile mural cells, classified as either smooth muscle or pericytes, that surround the entrance to a capillary from a metarteriole. They were originally discovered in the mesenteric circulation in the 1950s, and have also been found in the brain, where they regulate blood flow to the capillary bed. The sphincters can open and close the entrance to the capillary, with contraction causing changes in blood flow as vasomotion occurs. Vasomotion refers to the irregular, pulsating flow of blood through a capillary bed, which is regulated by chemical signals in response to changes in internal conditions such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels.
The function of precapillary sphincters is critical to maintaining proper blood flow. Normally, these sphincters are closed, only opening when the surrounding tissues need oxygen and have excess waste products. If all the precapillary sphincters in a capillary bed are closed, blood will bypass the capillary bed entirely and flow directly into the venous circulation. If the sphincter is damaged or unable to contract, blood can flow into the capillary bed at high pressures.
In the brain, precapillary sphincters control capillary flow while protecting the downstream capillary bed and brain tissue from adverse pressure fluctuations. Active nerve cells release vasodilators that increase their energy supply by dilating local blood vessels, a mechanism termed neurovascular coupling. This mechanism also forms the basis of BOLD functional neuroimaging signals.
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Disorders of the sphincter
Sphincters are ring-shaped muscles that open and close to regulate the flow of substances through passages in the body. They are found in the digestive tract, urinary tract, blood vessels, and eyes, and they are either under voluntary or involuntary control. When a sphincter malfunctions, it can lead to various disorders.
One such disorder is gastroesophageal reflux disease (GERD), which occurs when the lower esophageal sphincter fails to close completely or stay closed, allowing stomach acid to backflow into the esophagus. This can cause heartburn and other symptoms. Another disorder, dysphagia, is characterised by difficulty swallowing due to dysfunction of the upper esophageal sphincter.
Biliary reflux is another condition that occurs when the pyloric sphincter malfunctions, allowing bile to back up into the stomach. Similar to GERD, this disorder can cause stomach pain and other symptoms. Sphincter of Oddi dysfunction (SOD) is a condition that affects the flow of bile and pancreatic juice into the small intestine. It causes severe abdominal pain and can be treated with medications or a procedure called a sphincterotomy. SOD is more common in females between 20 and 50 who have had gallbladder surgery, although it can also occur in those with gallbladder dysfunction or other conditions such as pancreatitis or irritable bowel syndrome (IBS).
Biliary Sphincter Disorders (formerly known as SOD) include Biliary Sphincter Stenosis/Obstruction, which occurs when the sphincter mechanism is disturbed, causing a backup of bile and pancreatic juices, resulting in abdominal pain and jaundice. Functional Biliary Sphincter Disorder and Functional Biliary Sphincter Pain are other types of Biliary Sphincter Disorders, which may be caused by nerve damage during gallbladder surgery or other unknown factors affecting the function of the sphincter and the perception of pain.
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Frequently asked questions
A sphincter muscle is a circular muscle that normally 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.
The human body has many types of sphincter muscles, including the internal and external urethral sphincters, the lower oesophageal sphincter, the pyloric sphincter, the ileocecal sphincter, the sphincter of Oddi, the internal and external anal sphincters, and the orbicularis oculi muscle.
Sphincter muscles open and close to regulate the flow of substances like food, liquid, and waste through passages in the body. They are involved in excretion and prevent the backward movement of intraluminal contents.
When a sphincter malfunctions, it can lead to disorders such as gastroesophageal reflux disease (GERD), dysphagia, and biliary reflux. GERD occurs when the lower oesophageal sphincter doesn't shut completely, allowing stomach acid to backflow into the oesophagus. Dysphagia is caused by the dysfunction of the upper oesophageal sphincter, leading to difficulty swallowing. Biliary reflux occurs when bile backs up into the stomach due to a malfunctioning pyloric sphincter.
