There are three stages to swallowing: 1) the voluntary forcing of foodfrom the oral cavity into the pharynx; at this point swallowing becomes a reflex action; 2) the passing of food from the pharynx to the esophagus; pharyngeal nerves stimulate the trachea, eustachian tubes and the posterior opening of the nasopharynx to close while the tongue prevents food from reentering the mouth such that the only place left for the food to go is into the esophagus; 3) the passing of food from the esophagus to the stomach is aided by gravity. Muscular contractions (peristalsis) push food from one end of the esophagus to the other. This occurs through alternate contractions of circular and longitudinal muscles in the esophageal wall.
The last four centimeters of the esophagus is the cardiac sphincter, a muscular valve preventing the stomach contents from reentering the esophagus. Most of the esophagus (about 9 inches) lies within the thoracic cavity and is subject to subatmospheric pressures as are the lungs. The cardiac sphincter lies below the diaphragm and is subject to pressure which is 5 to 10 mm of Hg higher than that of the atmosphere. This higher pressure would force the stomach contents back into the esophagus if the cardiac sphincter were not present.