Anatomy And Physiology

The esophagus is a muscular tube approximately 20 to 25 cm long. The majority of the esophagus is located in mediastinum, posterior and slightly lateral to the trachea, with smaller cervical and abdominal components as well, as shown in Fig 71-1. There is an outer longitudinal muscle layer and an inner circular muscle layer. The upper third of the esophagus is made up of striated muscle. Distal to the upper third, smooth muscle appears as well, and the esophagus is all smooth muscle from the lower half down [including the lower esophageal sphincter (LES)]. The cells lining the esophagus are stratified squamous epithelial cells that have no secretory function.

FIG. 71-1. Anatomic relations of the esophagus (seen from the left side). The esophagus is about 25 cm (10 in.) long. The distance from the upper incisor teeth to the beginning of the esophagus (cricoid cartilage) is about 15 cm (6 in.); from the upper incisors to the level of the bronchi, 22 to 23 cm (9 in.); to the cardia, 40 cm (16 in.). Structures contiguous to the esophagus that affect esophageal function are demonstrated.

Two sphincters on either end regulate the passage of materials into and out of the esophagus. The upper esophageal sphincter (UES) prevents air from entering the esophagus and food from refluxing out of the esophagus into the pharynx. The LES regulates passage of food into the stomach and prevents stomach contents from refluxing into the esophagus. The UES is composed primarily of the cricopharyngeus muscle. Additional tone is variably provided by the inferior pharyngeal constrictor and the cervical esophagus.1 The UES has a resting pressure of around 100 mmHg. The LES is not discretely identifiable anatomically. The smooth muscle of the lower 1 to 2 cm of the esophagus, in combination with the skeletal muscle of the diaphragmatic hiatus, functions as the sphincter, with a resting pressure of 25 mmHg.2 The pressure within the resting esophagus itself closely approximates intrathoracic pressure. 3 Dysfunction of the LES is a major source of esophageal symptoms and is discussed below.

Three major anatomic constrictions exist within the esophagus and are important when considering esophageal foreign bodies and food bolus impaction, discussed below. They are located at the cricopharyngeus muscle, at the level of the aortic arch/left main-stem bronchus, and at the gastroesophageal junction. An empty, collapsed esophagus has no apparent constrictions: only with esophageal filling do the narrowings become apparent.

Innervation of the esophagus is mediated by both the sympathetic and parasympathetic systems and local nerve networks. Both Auerbach's and Meissner's plexuses are present in the esophagus, in the same distribution noted throughout the gastrointestinal tract. Reflex activity and homeostasis are also mediated by parasympathetic afferents and efferents, carried by the vagus nerve. Pain sensation from heat, spasm, distension, or chemical irritation of the esophagus travels exclusively through sympathetic nerve fibers. The heart has a pattern of innervation mirroring that of the esophagus, and there is a convergence of visceral and somatic stimuli within the sympathetic system. This is the anatomic basis that makes esophageal and cardiac chest pain notoriously similar, as discussed below. 4

The esophageal blood supply is derived from several arterial sources. The inferior thyroid artery, small branches of the thoracic aorta, and ascending branches form the left gastric and inferior phrenic arteries supply the esophagus throughout its length. The esophageal venous circulation includes a submucosal plexus of veins that drains into another plexus of veins surrounding the outside of the esophagus. Blood flows from this plexus into the inferior thyroid, azygos, coronary, and gastric venous systems. The last is one link between the portal and systemic venous systems. Variceal dilatation of the submucosal system can be responsible for massive upper gastrointestinal (GI) bleeding, reviewed below.

Swallowing is initiated voluntarily, then becomes reflex controlled. Control of the swallowing mechanism is provided by both central nervous system (CNS) input and local reflex feedback. As food is moved to the posterior pharynx during the pharyngeal stage of swallowing, the UES relaxes and the bolus passes into the esophagus. Generally this occurs within the first 2 s of swallowing. The food bolus in the upper esophagus subsequently reinitiates constriction of the UES. Peristalsis moves the food bolus down the esophagus (5 to 6 s) to the LES. The LES relaxes (but remains closed) with the onset of swallowing, remains relaxed for the 5 to 10 s it takes for the food bolus to make the trip down, and then recontracts with the peristaltic wave. 5 Simultaneous reflexes outside of the esophagus help to protect the pharynx and larynx from inadvertent reception of food. Impairment of the swallowing mechanism is reviewed below.

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