The lower esophageal sphincter (LES) is a circular band of muscle surrounding the esophagus at its lower end. It 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 or liquid has passed through, it serves as a “one-way valve,” letting food and liquid pass down into the stomach, but not from the stomach back up into the esophagus.
If the LES fails to remain adequately contracted, it can allow for acid reflux, leading to gastroesophageal reflux disease (GERD) or laryngopharynx reflux disease (LPRD). Alternately, if the muscle fails to relax appropriately when food or liquid reaches it en route to the stomach, the person has a condition called achalasia.