Sphincter

Sphincter

A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the human body, some microscopically small, in particular the millions of precapillary sphincters.

Types of Sphincters

Sphincters relax at death, often releasing fluids and faeces. There are two types of sphincters: one type is composed of skeletal muscle and is under voluntary control, and the other type is composed of smooth muscle and is under involuntary control.

Skeletal Muscle Sphincters

Sphincters composed of skeletal muscle are under voluntary control, which means they can be opened or closed at will. These include the:

• External anal sphincter
• External urethral sphincter

Smooth Muscle Sphincters

Sphincters composed of smooth muscle are under involuntary control, which means they cannot be opened or closed at will. These include the:

• Internal anal sphincter
• Internal urethral sphincter
• Lower esophageal sphincter
• Upper esophageal sphincter
• Pyloric sphincter
• Ileocecal sphincter

Function

The sphincters control the passage of liquids and solids. This is evident, for example, in the blowhole of a cetacean, where it allows the creature to be submerged for a long period of time, yet still take in air at the surface.

Pathology

Sphincter dysfunction can lead to various health problems and conditions. For instance, Achalasia is a disease where the lower esophageal sphincter fails to relax properly, making it difficult for food to enter the stomach.

See also

• Muscular system
• Digestive system
• Urinary system

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