The esophagus is a muscular tube connecting the throat (pharynx) with the
stomach. The esophagus is about 8 inches long, and is lined by moist pink
tissue called mucosa. The esophagus runs behind the windpipe (trachea) and
heart, and in front of the spine. Just before entering the stomach, the
esophagus passes through the diaphragm.
The upper esophageal sphincter (UES) is a bundle of muscles at the top of the
esophagus. The muscles of the UES are under conscious control, used when
breathing, eating, belching, and vomiting. They keep food and secretions from
going down the windpipe.
The lower esophageal sphincter (LES) is 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 are not under voluntary control.