Anatomically and functionally, the esophagus is the least complex section of the digestive tube. Its role in digestion is simple: to convey boluses of food from the pharynx to the stomach.
Like other parts of the digestive tube, it has four tunics (layers), but important differences exist in the composition of these tunics in comparison to more distal sections of the tube. First, instead of the muscular tunic being entirely smooth muscle, as it is in the stomach and intestines, the wall of the esophagus contains a variable amount of striated muscle. In dogs, cattle and sheep, its entire length is striated muscle, whereas in cats, horses and humans, the proximal esophagus has striated muscle and the distal esophagus smooth muscle. Second, instead of the esophagus being free as it courses through the thoracic cavity, it is embedded in the connective tissue; thus, its outer tunic is referred to as adventitia instead of serosa.
In its role as the first conduit in the digestive tube, the esophagus is routinely exposed to rough and abrasive foodstuffs, like fragments of bone, fibrous plant leaves and Doritos. Its surface should therefore be resistant to trauma, and indeed, the esophagus is lined with stratified squamous epithelium, as seen below in an image from a cat's esophagus:
Absorption in the esophagus is virtually nil, but the mucosa does contain a few mucous glands that, as you might expect, secrete mucus to aid in lubrication.
The body of the esophagus is bounded by physiologic sphincters known as the upper and lower esophageal sphincters. The upper sphincter is composed largely of a muscle that is closely associated with the larynx. When relaxed, as it is during swallowing, this muscle pulls the larynx forward and aids in routing food into the esophagus instead of the larynx. The lower sphincter is simply the muscle that surrounds the esophagus just as it enters the stomach. Both the upper and lower sphincter are closed except during swallowing, which prevents constant entry of air from the oral cavity or reflux of stomach contents.
During swallowing, boluses of food are propelled through the esophagus by strong peristaltic contractions. In dogs and humans, it takes 4-5 seconds for the bolus to traverse the esophagus. If the bolus is not delivered in "one pass", secondary waves of peristalsis are initiated at the point of distention, which almost always result in delivery of the bolus to the stomach. Congenital and acquired disorders in esophageal motility that interfere with this usually reliable delivery of food are rather common in both animals and man.
Source: Republished with permission by Richard Bowen - Hypertexts for Biomedical Sciences