The esophagus is a muscular tube that transports ingested material from the pharynx to the stom­ach.

In the resting state the esophagus is collapsed; however, it is capable of distending to accommo­date passage of both fluid and solid materials. It is divided into three sections—the cervical, the tho­racic, and the short abdominal portions—and it is bounded at each end by sphincters.

The uEs remains closed at all times, opening only to allow passage of a bolus. It closes promptly after the bolus is passed. The duration of opening of the uEs is determined by a central neural mechanism that senses the volume of the bolus that is being propelled aborally by the pharynx. The state of consistent closure of the uEs main­tains a high-pressure zone that serves as an impor­tant defense mechanism, helping protect against esophagopharyngeal reflux and aspiration of ingesta.

The lower esophageal sphincter (LEs) or gas­troesophageal junction (GEJ) comprises the junc­tion between the esophagus and stomach. The LEs functions as a physiologic sphincter. It acts as a zone of high resting pressure that promotes unidi­rectional flow from the esophagus to the stomach and helps prevent reflux of gastric contents into the esophagus. In dogs the LEs consists of an outer layer of striated muscle and an inner layer of smooth muscle, whereas in cats the LEs is com­posed entirely of smooth muscle. Mechanisms for prevention of gastroesophageal reflux include a variety of anatomic factors in addition to the LEs itself. The entire abdominal segment of the esoph­agus, as well as the surrounding structures, plays a role.

The LEs opens and closes in response to neural activity (vagal tone) associated with the swallowing mechanism. Its function can also be influenced, however, by hormones, by drugs (e.g., acepro- mazine, atropine, diazepam, propofol, xylazine, halothane, and isoflurane decrease LEs pressure, whereas metoclopramide, cisapride, bethanechol, erythromycin, and domperidone increase LEs pres­sure), and by local events such as inflammation. In response to esophageal peristaltic contractions, the LEs undergoes a phase of initial relaxation that is followed by postdeglutition contraction. Initial relaxation begins when an esophageal peristaltic contraction occurs in the proximal esophagus. Postdeglutition contractions prevent reflux of a food bolus following its passage into the stomach.

Boluses are moved through the esophagus by a series of strong, well-coordinated contractions, which are produced by intense muscular activity. The esophagus of the dog is relatively longer than that of humans. Animals rely much more on propulsive esophageal activity than do humans, in whom gravity also plays a significant role in move­ment of material through the esophagus. In fact, it has been shown that the canine esophagus is able to develop 10 times the pressure that develops in the human esophagus.

The swallowing process has been divided into three major phases: oropharyngeal, esophageal, and gastroesophageal. Primary esophageal contractions are triggered by the oropharyngeal phase of swal­lowing. Secondary peristaltic waves occur in response to the effects of esophageal luminal dis­tention and tactile stimuli. These waves begin proximal to the bolus. Progression of primary and secondary peristaltic waves depends on the size and the location of boluses present in the esopha­gus.

The esophagus has four distinct layers: the adventitia, the muscularis, the submucosa (which contains glands, nerves, and blood vessels), and the mucosa. With no serosal layer present, the submu­cosal layer of the esophagus is considered to have the greatest holding strength when sutured. In dogs, the muscle layer of the esophagus is com­posed entirely of striated muscle. In cats, the cra­nial two thirds of the esophagus is striated muscle, and the distal one third is smooth muscle. The only smooth muscle in the esophagus of a dog is the muscularis mucosa. The muscularis mucosa has minimal contribution to peristaltic activity. The esophagus receives its innervation from the sym­pathetic and the vagus nerves, including the recur­rent laryngeal branches. The vagal supply is the more important of the two. Branches of the thy­roid arteries supply blood to the cervical esopha­gus; the bronchoesophageal arteries supply the cranial portions of the thoracic esophagus, and the branches of the aorta, the intercostals, and the gas­tric arteries supply the remaining portion of the esophagus.

The most common esophageal disorders that are seen in clinical practice are megaesophagus, esophagitis, esophageal strictures, and esophageal foreign bodies. These problems are discussed in detail in this chapter. Less common problems that are discussed include vascular ring anomalies, hiatal disorders, and esophageal neoplasia. A list of esophageal diseases based on signalment appears in Table 4-1.