The esophagus is the tube that carries food and liquids from the mouth into the stomach. The wall of the esophagus is composed of layers of muscle that propel food to the stomach by rhythmic waves of contractions called peristalsis. There are muscle rings at each end of the esophagus, – called sphincters. These sphincters open to allow food to move forward and then close to prevent food and acid from moving backward, helping prevent acid reflux and aspiration or choking.
Upon swallowing, the upper esophageal sphincter opens and the esophageal muscles contract to push food downward. The lower esophageal sphincter opens temporarily to allow food to pass into the stomach. Then the sphincter closes to prevent food and stomach acid from flowing back up the esophagus.