Sphincter

A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and relaxes in a controlled manner as required by normal physiological functioning. Sphincters are found throughout the animal kingdom and are especially numerous and functionally diverse in humans. In the human body alone, there are more than sixty recognised sphincters, ranging from large, easily identifiable muscles to microscopic structures such as the millions of precapillary sphincters that regulate blood flow at the tissue level. Together, these muscles play an essential role in digestion, circulation, respiration, excretion, reproduction, and sensory regulation.

Structural and physiological basis

Each sphincter is associated with a lumen, the hollow interior of a tubular structure such as the gastrointestinal tract, a blood vessel, or a duct. When the sphincter muscle is contracted, its fibres shorten and thicken, causing the lumen to narrow or close. When the muscle relaxes, its length increases, the muscle ring loosens, and the lumen opens, allowing the controlled passage of liquids, solids, or gases.
This basic mechanism is conserved across a wide range of biological systems. For example, sphincter control is evident in the blowhole of marine mammals, where coordinated muscular relaxation allows breathing at the water’s surface while remaining tightly sealed at other times to prevent water ingress. In humans, sphincters operate continuously and often subconsciously, adjusting to internal pressures, chemical signals, and neural commands.
At death, sphincters typically lose neural input and muscle tone, leading to relaxation and the possible release of bodily fluids and faecal matter, highlighting their dependence on active physiological control.

Role in digestion and internal transport

Sphincters are particularly prominent in the digestive system, where they regulate the orderly movement of food and digestive secretions while preventing harmful backflow. They ensure that digestion proceeds in a unidirectional and compartmentalised manner, with each region of the alimentary canal performing its specialised function.
A key example is the lower oesophageal sphincter (also known as the cardiac sphincter), located at the junction of the oesophagus and the stomach. This sphincter remains closed most of the time, preventing acidic gastric contents from refluxing into the oesophagus, yet relaxes transiently to allow swallowed food to enter the stomach. Dysfunction of this sphincter is a major contributor to gastro-oesophageal reflux disease.
Further along the digestive tract, the pyloric sphincter controls the release of partially digested food from the stomach into the duodenum, while the ileocaecal valve regulates flow between the small and large intestines, limiting the backward movement of colonic contents into the ileum. These sphincters coordinate with peristaltic movements to optimise digestion and absorption.

Classification of sphincters

Sphincters can be classified in more than one way, reflecting differences in structure, control, and function.

Anatomical and functional sphincters

Anatomical sphincters possess a distinct, localised thickening of circular muscle fibres. This structural specialisation allows them to act as clearly defined valves, often identifiable on gross anatomical examination. Examples include the pyloric sphincter and the internal anal sphincter.
Functional sphincters, by contrast, lack a discrete muscular ring. Instead, their sphincteric action is achieved through coordinated contraction of surrounding muscles or intrinsic muscle layers within the organ wall. Although less obvious anatomically, functional sphincters are no less important and rely heavily on neural and physiological regulation to maintain closure and relaxation at appropriate times.

Voluntary and involuntary control

Sphincters are also classified according to the type of nervous control that governs their activity.

• Voluntary sphincters are supplied by the somatic nervous system and can be consciously controlled. These sphincters are composed of skeletal muscle and allow deliberate regulation of bodily functions, such as urination and defecation.
• Involuntary sphincters are controlled by the autonomic nervous system and consist primarily of smooth muscle. Their activity is regulated subconsciously in response to reflexes, hormones, and local chemical conditions.

Some regions, notably the anal canal, incorporate both voluntary and involuntary sphincters working together to provide fine control and social continence.

Major sphincters in the human body

Sphincters are distributed throughout multiple organ systems, each adapted to the functional demands of its location.
In the eye, the sphincter pupillae is a smooth muscle within the iris that constricts the pupil in response to bright light, protecting the retina and contributing to visual acuity. Surrounding the eye, the orbicularis oculi muscle functions as a sphincter that enables blinking and eyelid closure, safeguarding the eye and spreading tear fluid across the cornea.
Within the digestive system, several sphincters are critical to normal physiology:

• The lower oesophageal sphincter at the entrance to the stomach
• The pyloric sphincter at the stomach’s exit
• The sphincter of Oddi (also called Glisson’s sphincter), which regulates the flow of bile and pancreatic secretions into the duodenum
• The ileocaecal valve, limiting reflux between intestinal segments

In the urinary system, the urethral sphincter controls the outflow of urine. It includes both involuntary smooth muscle and voluntary skeletal muscle components, enabling continence while allowing conscious voiding.
At the anus, two distinct sphincters regulate defecation:

• The internal anal sphincter, composed of smooth muscle and under involuntary control
• The external anal sphincter, composed of skeletal muscle and under voluntary control

The coordination between these two muscles is essential for maintaining continence and allowing controlled bowel movements.
The orbicularis oris, a complex muscular arrangement encircling the mouth, functions as a sphincter that enables speech articulation, facial expression, and control of food intake.

Microscopic sphincters and circulation

Not all sphincters are large or easily observed. Precapillary sphincters are microscopic rings of smooth muscle located at the junction between arterioles and capillaries. These structures regulate blood flow into capillary beds in response to local metabolic activity.
When tissue metabolism increases, precapillary sphincters relax, allowing greater blood flow and improved delivery of oxygen and nutrients. Conversely, when metabolic demand is low, they remain constricted, diverting blood to more active tissues. This mechanism is fundamental to efficient circulation and tissue homeostasis.

Neural and chemical regulation

Sphincter activity is governed by a complex interplay of neural signals, hormones, and local chemical conditions. Autonomic reflexes can rapidly alter sphincter tone in response to stretch, pressure, or chemical stimuli, while higher brain centres can exert voluntary control over selected sphincters.
Neurotransmitters such as acetylcholine and nitric oxide, as well as hormones involved in digestion, play important roles in modulating smooth muscle relaxation and contraction. Disruption of these regulatory mechanisms can lead to clinical disorders, including reflux disease, incontinence, and obstructive syndromes.