The esophagus is a hollow muscular tube that connects the back of the throat to the top of the stomach. In an adult, this tube normally ranges from about 10-14 inches in length, and one inch in diameter. At rest, the esophagus is closed but opens readily to accept food and liquids. The muscles in the upper portion of the esophagus are under voluntary control. The remaining portion consists of smooth muscle like the rest of the digestive tract and is not under voluntary control.
To keep food from coming back up from the stomach, the esophagus has two circular bands of involuntary muscle. The one at the top of the esophagus is called the upper esophageal sphincter (UES) and the one at the bottom of the esophagus (which separates the esophagus from the stomach) is called the lower esophageal sphincter (LES). The area in between is known as the body of the esophagus.
The act of swallowing takes a great deal of coordination, most of which is unconscious. When food or drink is placed into a person's mouth, the body's nervous system detects this and the brain directs the UES to open allowing the food or drink to pass into the esophagus (although this can sometimes be affected by strokes and other injuries).
When swallowing, sensors within the smooth muscles of the esophagus, which detect stretch from the contents swallowed, tell the brain to make those muscles contract in order to push the contents downward. At this point, the LES relaxes to allow the food or drink to enter the stomach. This entire series of events takes about eight seconds.
After swallowing, the muscles in the esophagus may continue to contract in order to sweep remaining material into the stomach. This is also a protective mechanism to strip any acid that may have washed back into the esophagus from the stomach.