What Is a Sphincter?

Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances, such as bile, urine, and feces, through the body. Although many are found in the gastrointestinal (GI) tract, there are also sphincters in the urinary tract and blood vessels. There are even those in the eyes 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.

This article describes what various sphincters of the GI tract, urinary tract, eyes, and blood vessels do. It also explains what can happen if they don't work.

Sphincters of the GI Tract

Six distinct sphincters are situated within the GI tract (also known as the digestive system). Some consist of smooth muscles that function involuntarily or by reflex, while others are made of striated muscles that function voluntarily.

Upper Esophageal Sphincter

The upper esophageal sphincter (UES), also known as the inferior pharyngeal sphincter, is an involuntary sphincter found at the end of the pharynx (throat).

The UES protects the entrance to the esophagus, which carries food from your throat to your stomach. The swallowing reflex triggers the opening of this sphincter.

The UES prevents air from getting into the esophagus while you breathe. It also prevents you from breathing food into your lungs. Because of its location, the UES also plays a role in burping and vomiting.

Malfunctioning of the UES can cause problems swallowing, which is known medically as dysphagia. The UES can also be affected by a chronic (persistent) acid reflux called gastroesophageal reflux disease (GERD).

Lower Esophageal Sphincter

The lower esophageal sphincter (LES), also known as the cardiac sphincter, is an involuntary sphincter located at the bottom of the esophagus, where it connects with the stomach. (The term cardiac in this context does not refer to the heart.)

The LES lets food pass from the esophagus into the stomach and allows air to escape from the stomach when burping. It also prevents stomach acid from washing back up into the esophagus. A malfunction of the LES is one of the primary causes of GERD.

Pyloric Sphincter

The pyloric sphincter is located between the stomach and the first part of the small intestine, called the duodenum. The pyloric sphincter opens to allow partially digested food to pass from the stomach into the duodenum, where it will be exposed to bile and other digestive enzymes.

A malfunction of the pyloric sphincter can cause bile (a digestive enzyme produced by the liver) to back up into the stomach and sometimes the esophagus. Symptoms of bile reflux are indistinguishable from those of acid reflux.

Sphincter of Oddi

The sphincter of Oddi (SO) is an involuntary sphincter located where the pancreatic duct and common bile duct are connected to the duodenum.

The pancreatic duct delivers digestive enzymes from the pancreas that then mix with bile delivered from the liver via the common bile duct. The SO ensures that the right amounts are released when food passes into the duodenum.

Sphincter of Oddi dysfunction (SOD) is a condition where this SO doesn't work as it should. This causes bile and pancreatic enzymes to back up into the stomach. A relatively rare disorder, SOD can cause waves of stomach pain along with severe chest pain.

Ileocecal Sphincter

The ileocecal sphincter is an involuntary sphincter located where the small intestine and large intestine (colon) meet.

Also known as the ileocecal valve, not much is known about this sphincter. 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.

Anal Sphincter

The anal sphincter is located at the opening of the rectum at the end of the digestive tract. The anal sphincter regulates defecation (the evacuation of stool from the body).

The anal sphincter has both inner and outer muscles:

• The inner sphincter is involuntary and prevents stool from leaking out.
• The outer sphincter is predominantly voluntary and allows you to pass or hold stool on demand.

A malfunction of the anal sphincter can cause fecal incontinence (the loss of bowel control).​

Urethral Sphincter

The urethral sphincter, also known as the sphincter urethrae, is the sphincter that controls the passing of urine from the body.

Like the anal sphincter, the urethral sphincter has inner and outer muscles that regulate urine flow through the urethra (the narrow tube through which urine—and semen in people with a penis—exits the body). The inner muscle has involuntary control, and the outer muscle has voluntary control.

Dysfunction of the urethral sphincter can lead to urinary incontinence (the loss of bladder control).

Precapillary Sphincters

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 in the body, called capillaries.

There are millions of these microscopic sphincters located throughout the body, including those that service capillaries of the brain. They are important because they ensure a consistent flow of blood—and consistent pressure within the vessels—irrespective of natural variations in a heartbeat.

If precapillary sphincters don't work as they should, blood can flow freely into the capillaries and create excessive pressure. This can cause fluids to leak out of the capillary walls, leading to edema (fluid buildup) in places like the legs (peripheral edema), lungs (pulmonary edema), or brain (cerebral edema).

Iris Sphincter

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.

Two different muscles respond to changes in light (known as the pupillary light reflex) that control the constriction and dilation (widening) of the pupils:

• The iris sphincter muscles around the pupils' rim are responsible for constriction.
• The iris dilator muscles radiating outward from the pupils are tasked with dilation.

When the iris sphincter malfunctions due to medications or other causes, it can cause vision blurred vision and light sensitivity due to miosis (abnormally small pupils). The same can occur if the iris dilator muscles malfunction, causing mydriasis (abnormally large pupils).

Summary

Sphincters are circular muscles that open and close passages in the body. Those in the gastrointestinal (GI) tract help regulate the flow of food from the esophagus to the anus as well as the flow of bile and digestive enzymes into the intestine.

A malfunction of the GI sphincters can lead to problems such as acid reflux, gastroesophageal reflux disease (GERD), and fecal incontinence.

There are sphincters in other parts of the body as well. They include the urethral sphincter, which controls the flow of urine; the iris sphincters, which narrow the pupils; and the precapillary sphincters, which help regulate the flow of blood through smaller vessels.