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. The esophagus begins as an extension of the pharynx in the back of the oral cavity. It then courses down the neck next to the trachea, through the thoracic cavity, and penetrates the diaphragm to connect with the stomach in the abdominal cavity.
Like other parts of the digestive tube, the esophagus has four tunics, 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. The mucosa does contain mucous glands that are expressed as foodstuffs distend the esophagus, allowing mucus to be secreted and 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 esophageal sphincter is the muscle that surrounds the esophagus just as it enters the stomach.
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. In humans, common disorders involving the esophagus include heartburn and gastroesophageal reflux disease (GERD). In both cases, the lower sphincter does not close properly, allowing acid from the stomach to reflux back into the esophagus, causes a burning sensation in the chest or throat (heartburn) or additional signs such as coughing, coughing or a sensation of choking.
An associated problem is acid indigestion, which occurs when refluxed stomach acid is tasted. Occasional heartburn is very common, but if it occurs more than a time or two each week, it could signify a more serious problem that requires treatment, usually with dietary managment and drugs that suppress secretion of gastric acid.
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