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How Many Sphincter Muscles Are In The Human Body

How Many Sphincter Muscles Are In The Human Body? And Functions

The human body is a complex system of numerous organs, tissues, and muscles that work together to maintain optimal functioning. One interesting aspect of the human body often overlooked is the sphincter muscles. 

While we might not give them much thought in our daily lives, they play vital roles in our digestive, urinary, and respiratory systems and various other parts of our anatomy.

So that’s why it’s important to know how many sphincter muscles are in the human body to appreciate the various components in our body. Let’s start our discussion by answering the question.

How Many Sphincter Muscles Are In The Human Body?

It is estimated that there are approximately 50 sphincter muscles distributed throughout various organs and systems. 

From the digestive to the urinary and reproductive systems, these sphincter muscles ensure proper functioning and prevent unwanted leakage or backflow. 

What Is A Sphincter Muscle?

Sphincter muscles are ring-like muscles that surround various openings in the body, acting as gates to regulate the flow of substances. 

These muscles control the movement of materials through different body systems by contracting and relaxing.

Some sphincters, like the anal sphincters, are under voluntary control, allowing us to initiate their action consciously. 

Others, like the lower esophageal sphincter and many in the digestive system, function involuntarily, responding to various stimuli to regulate the movement of substances through organs and systems.

1. Sphincter Muscles In The Digestive Tract

The digestive tract is a complex system that allows us to break down and absorb nutrients from our food. 

Within this system, several sphincter muscles play an important role in regulating the movement of food and digestive juices. They include:

  • Upper Esophageal SphincterIt is located at the top of the esophagus, separating the esophagus from the pharynx and larynx. 

The primary function is to prevent air from entering the esophagus during breathing and to keep stomach acid from regurgitating into the throat. 

This sphincter relaxes when you swallow food or liquid, allowing it to pass into the esophagus and stomach.

  • Lower Esophageal Sphincter – As the name suggests, it is at the lower end of the esophagus, where it meets the stomach. Its primary role is preventing stomach acid backflow into the esophagus. 

This sphincter opens when we swallow and closes tightly to keep stomach contents from moving backward.

  • Pyloric Sphincter – Moving further along the digestive tract, there’s the Pyloric Sphincter. 

This sphincter is found at the junction between the stomach and the small intestine. It controls the flow of partially digested food from the stomach into the small intestine. 

The Pyloric Sphincter opens and closes gradually to allow small amounts of food to pass through, ensuring proper digestion and absorption.

  • Ileocecal Sphincter – The Ileocecal Sphincter is found at the junction between the small and large intestines. 

Its primary function is to prevent backflow of waste material from the large intestine into the small intestine. 

It also helps regulate the movement of waste through the digestive system, allowing for efficient absorption of water and nutrients in the small intestine before elimination.

  • Sphincter of Oddi – The muscle is found at the point where the common bile duct and pancreatic duct enter the small intestine. 

This sphincter regulates bile flow from the liver and gallbladder and digestive enzymes from the pancreas into the small intestine. 

It ensures that these substances are released appropriately at the right time for optimal digestion.

2. Anal sphincters

The anal sphincters are perhaps some of the most well-known sphincters in the human body. These muscles are located in the region surrounding the anus, forming a tight ring. There are two main types:

  • Internal anal sphincter – This inner sphincter is made up of smooth muscle, and its function is involuntary. It remains contracted most of the time, helping to prevent the accidental release of feces. 

When stool enters the rectum, it causes reflex relaxation of this sphincter, signaling the need for a bowel movement.

  • External anal sphincter – The outer sphincter comprises skeletal muscle, which means it is under conscious control. 

It allows us to voluntarily contract or relax it, enabling us to delay a bowel movement until it’s convenient.

3. Urethral sphincters

Urethral sphincters serve as guardians of the urinary tract, controlling the release of urine from the bladder. There are also two primary types:

  • Internal urethral sphincter – Similar to the internal anal sphincter, this sphincter consists of smooth muscle and operates involuntarily. It stays contracted to prevent urine from constantly leaking from the bladder.
  • External urethral sphincter – Just as with the external anal sphincter, the external urethral sphincter is made of skeletal muscle. 

It allows us to control the flow of urine voluntarily. When you decide to urinate, this sphincter relaxes, allowing urine to pass through the urethra. Together, these urethral sphincters ensure you can hold and release urine appropriately.

4. Precapillary Sphincters

Moving from the more familiar anal and urethral sphincters into the circulatory system, you will find the precapillary sphincters. 

These tiny muscles are found at the entrance of capillaries, the smallest blood vessels. Their function is crucial for regulating blood flow to different tissues.

Precapillary sphincters can constrict or dilate, controlling the amount of blood that flows through the capillaries. 

When a specific tissue requires more oxygen and nutrients, these sphincters relax, allowing a greater blood volume to enter the capillaries. 

Conversely, when less blood is needed in a particular area, they constrict to redirect blood flow elsewhere.

5. Iris Sphincters

The iris sphincters are within our eyes. The iris is the colored part of the eye surrounding the pupil. There are two sets of iris sphincter muscles:

  • Pupillary Constrictor – When this sphincter contracts, it causes the pupil to constrict or become smaller. This response occurs in bright light to limit the amount of incoming light.
  • Pupillary Dilator – Conversely, contracting the pupillary dilator muscle can enlarge the pupil. It happens in dim light conditions to allow more light into the eye for better vision.

What Is The Difference Between A Valve And A Sphincters?

Valves and sphincters are involved in regulating the flow of substances but differ in their design, function, location, and control mechanisms. Here is a table to compare the two.

Valve Sphincter
What is it? A valve is a mechanical or biological device that regulates the flow of fluids (liquids or gases) in a specific direction. A sphincter is a specialized circular muscle that controls the opening and closing of passages or orifices in the body.
Function Valves primarily control the direction of fluid flow, allowing it to move in one direction while preventing backflow. Sphincters regulate the passage of substances, such as food, air, or bodily waste, through various body parts by opening and closing as needed.
Locations Valves in the human body are in the heart. Sphincters are located within the human body, serving functions related to digestion, urination, and other bodily processes.

What Happens If A Sphincter Fails To Work?

A sphincter failing to work properly can lead to various health issues and disruptions in the body’s normal functioning. 

Sphincters act as gatekeepers, controlling the flow of substances in and out of certain body parts. And if a given sphincter muscle is not working correctly, that can be a problem.

For instance, the lower esophageal sphincter separates the esophagus from the stomach. If the LES doesn’t work correctly, stomach acid can flow back into the esophagus, causing heartburn or gastroesophageal reflux disease (GERD). 

Another perfect example is the anal sphincter, which controls the release of feces from the rectum. If this sphincter fails to work properly, it can result in fecal incontinence, where a person can’t control bowel movements. 

In the urinary system, the urinary sphincters help control the release of urine from the bladder. If these sphincters malfunction, it can lead to urinary incontinence, causing unintentional urine leakage. The self-esteem issues that come with such problems are unbearable.


How many sphincter muscles are in the human body? Experts estimate that approximately 50 sphincter muscles are present in the human body. 

The human body has various sphincter muscles that serve different functions and play important roles in maintaining bodily functions. 

These muscles, including the digestive, respiratory, urinary, and reproductive systems, can be found throughout the body. They help regulate the flow of food, air, urine, and feces.

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