Editing Sphincter (section)

==Functioning==
Each sphincter is associated with the lumen (opening) it surrounds. As long as the sphincter muscle is contracted, its length is shortened and the lumen is constricted (closed). Relaxation of the muscle causes it to lengthen, opening the lumen and allowing the passage of liquids, solids, or gases.

This is evident, for example, in the [[Blowhole (biology)|blowhole]]s of numerous [[marine mammal]]s.

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]], is closed most of the time, keeping [[stomach acid|acids]] and other stomach contents from pushing up and into the oesophagus, but opens to let swallowed food pass into the stomach.