The esophagus may be a muscular tube connecting the throat (pharynx) with the stomach. The esophagus is about 8 inches long, and is lined with a moist pink tissue called mucosa. The esophagus runs behind the windpipe (trachea) and heart, and ahead of the spine. Before entering into the stomach, the esophagus passes through the diaphragm.
The upper esophageal sphincter (UES) may be a bundle of muscles at the highest of the esophagus. The muscles in the UES are controlled by under consciousol when we breathing, eating, belching, and vomiting. They keep food and secretions from taking place the windpipe.
The lower esophageal sphincter (LES) may be a bundle of muscles at the low end of the esophagus, where it meets the stomach. When the LES is closed, it prevents acid and stomach contents from traveling backwards from the stomach. The LES muscles aren't under voluntary control.