What keeps air from entering the esophagus?

The primary structure that prevents air from entering the esophagus is the upper esophageal sphincter (UES), a muscular ring located at the top of the esophagus. It remains closed during breathing to prevent air from entering the esophagus and directs food and liquids into the stomach during swallowing. Additionally, the coordinated action of swallowing muscles and the pressure difference between the thoracic cavity and the esophagus also help keep air out.