A circular band of muscle surrounding the esophagus at its lower end. This muscle should be in a state of continual contraction, relaxing only momentarily to allow food to pass into the stomach. Given that the muscle should immediately contract again once food/liquid has passed through, it serves as a “one-way valve” to allow food and liquid to pass into the stomach, but not backwards from stomach into the esophagus. When the LES fails to remain adequately contracted, it can allow for [intlink id=”21″ type=”post”]acid reflux[/intlink] leading to [intlink id=”189″ type=”post”]gastroesophageal reflux disease[/intlink] (GERD) and [intlink id=”294″ type=”post”]laryngopharynx reflux disease[/intlink] (LPRD). If the muscle fails to relax appropriately when the food/liquid bolus reaches it in its travel toward the stomach, the person has a condition called achalasia.