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. Sphincters relax at death, often releasing fluids and faeces.
Functions.
Sphincters control the passage of liquids, solids, and gases. This is evident, for example, in the blowholes of numerous marine mammals.
Many sphincters are used every day in the normal course of digestion. For example, the lower oesophageal sphincter (or cardiac sphincter), which resides at the top of the stomach, keep stomach acids and other stomach contents from pushing up and into the oesophagus. During contraction of sphincter/circular muscles, the lumen (opening) associated with the sphincter constricts (closes). This constriction is caused by the shortening of the sphincter muscle. Relaxation of a sphincter muscle causes it to lengthen, opening the lumen.
Classifications.
Sphincters can be further classified into functional and anatomical sphincters:
Sphincters can also be voluntarily or involuntarily controlled: