The esophagus conveys ingested material from the mouth to the stomach. It is one of the simpler regions of the gastrointestinal (GI) tract; a roughly 8-10 inch muscular tube that runs from the back of the oral cavity (i.e., pharynx), through the chest cavity, and into the abdomen where it joins with the opening of the stomach (i.e., cardia). After ingested materials have traveled down the esophagus, they are emptied into the acidic environment of the stomach for chemical and mechanical digestion.
While the thick cellular layer of the stomach is a suitable barrier against stomach acid, the thinner mucous membrane of the esophagus was not designed to withstand such harsh conditions. To protect the esophagus from the potential back-flow of stomach contents (reflux), a sphincter is located at the junction between the esophagus and stomach, called the gastroesophageal or lower esophageal sphincter (LES). This sphincter, a circular band of thickened muscle, surrounds the lower esophagus and pinches it closed. The LES is usually closed. It opens to allow the passage of swallowed food or drink, a reflex that is triggered by the act of swallowing.
Aiding the closure of the LES is the diaphragm (a wide, flat muscle that helps to expand the lungs during respiration). Internally, the diaphragm separates the chest cavity from the abdomen, and the esophagus passes through a hole in the diaphragm (called the hiatus) on its way from the mouth to the stomach. The LES is situated near the part of the esophagus that passes through the diaphragm, so that contraction of the diaphragm can reinforce the closure of the sphincter (Kuo 2006).