The esophagus

After the food is ground by the teeth, lubricated by the saliva and made into a soft pulp in the mouth, it is carried from the throat to the stomach by a muscular tube, the esophagus. This process is called swallowing. Swallowing is triggered by a signal transmitted by the vagus nerve. (The vagus is the tenth cranial nerve that wanders all the way down from the brainstem to the colon. It supplies nerve fiber to many parts of the digestive system and brings sensory information back to the brain.) The timing of the swallowing sequence is dependent on nerves intrinsic to the esophagus.
The approximately 25 cm long esophagus begins as an extention of the pharynx, courses down the neck next to the trachea (windpipe) and penetrates the diaphragm (the principal muscle of the respiratory system that separates the abdominal and thoracic cavities) to connect with the stomach in the abdominal cavity.
The esophagus propels food toward the stomach by powerful waves of muscle contractions passing through its walls. This motion of the muscles is called peristalsis.
The upper third of the esophagus consists of striated muscle, while the lower two-thirds consist of smooth muscle.
The esophagus, being the first conduit of the digestive system after the mouth, is often exposed to rough and abrasive food parts, as well as extreme temperature. Its surface must therefore be resistant of trauma. For its protection the inner wall of the esophagus is lined with stratified squamous epithelium. The epithelium is a primary membranous tissue composed of one or more layers of cells forming the covering of most internal and external surfaces of the body and its organs. Squamous epithelial cells are flat and often overlapping, they compose the outer layer of skin and line certain internal cavities of body organs.
The esophagus is bounded by physiologic sphincters: the upper and lower esophageal sphincters. The upper sphincter, located next to the larynx, contains a muscle which pulls the larynx forward when swallowing, in order to rout the food into the esophagus instead of the larynx. The lower esophageal sphincter, also known as cardiac sphincter, is the muscle that surrounds the esophagus just as it enters the stomach. The lower esophageal sphincter relaxes before the esophagus contracts, and allows food to pass through to the stomach. After food passes into the stomach, the sphincter constricts to prevent the contents from regurgitating into the esophagus.
Normally, the upper and lower sphincters are closed except during swallowing, which prevents constant entry of air from the oral cavity or reflux of stomach contents. When the bottom muscles weaken, the cardiac sphincter does not close properly, allowing stomach contents and stomach acid to reflux back into the esophagus, causes a burning sensation in the chest or throat. This is a common disorder called heartburn or gastroesophageal reflux disease.