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Updated: 03/12/08 02:21 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Author: C.E. Pope ♦
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C. Structure of the Esophagus
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1. Gross Anatomy
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Musculature Upper 1/3 = striated Lower 2/3 = smooth Fixed at origin and termination ~25cm in length |
The human esophagus is a muscular tube composed of both striated (upper third) and smooth muscle (lower two-thirds) running from the oropharynx through the posterior mediastinum which is within the thorax, to the stomach, which is within the abdominal cavity. In the resting state it is collapsed; it only opens to accommodate swallowed or regurgitated material. Its main areas of fixation are at its origin and termination. It is approximately 30 mm in its longer lateral diameter and 19 mm in its shorter anterior-posterior diameter, although it has a capability of distending to greater dimensions. Its length is approximately 25 cm. Both ends of the esophagus are modified in order that the lumen of the esophagus can be shut off from the oropharynx at its upper end and from the stomach at its lower end.
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| Upper esophageal sphincter (UES) |
The upper esophageal sphincter (UES) is located at the upper end of the esophagus, and separates the pharynx from the esophagus. The cricopharyngeal muscle has oblique fibers that form a muscular sling that inserts anteriorly on the cricoid cartilage. Normally, this muscle is constantly contracted which closes the mouth of the esophagus. A sense of resistance is met in this area when one attempts to pass a tube into the esophagus. Fish bones and other inadvertently swallowed objects most commonly become lodged at the level of the upper sphincter; they may also get caught lateral to the esophageal opening in either of two closed pockets - the pyriform sinuses; thus passage of a tube through this zone should always be gentle and midline lest the blind ends of these sinuses be perforated.
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| Lower esophageal sphincter (LES) |
The lower end of the esophagus also has a closing mechanism which separates the lumen of the esophagus from that of the stomach. For many years, most workers believed that there was no anatomic evidence of a specialized muscle bundle at the lower end of the esophagus which could function as a sphincter. Modern studies indicate that there is a slight thickening of the circular muscle, and a different arrangement of the muscle bundles in this zone. Physiologically, the muscle from this zone in humans has different mechanical and pharmacological characteristics than the muscle above it in the esophagus and below it in the stomach.
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| Esophageal narrowings |
The esophagus, in its course through the mediastinum, has several
narrowings. The first is the upper esophageal sphincter and the next is the region of aortic crossing where the transition from striated to smooth muscle is usually found. Narrowed areas can be found where the left mainstem bronchus crosses and where the left atrium lies next to the esophagus. The last narrowing is in the zone of the lower esophageal sphincter.
These narrowed areas can be recognized radiologically when the esophagus is filled with barium. |
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| Esophagus traverses the diaphragm through the diaphragmatic hiatus |
The esophagus must traverse the diaphragm in order to reach the stomach. A seal must be maintained between the esophagus and the surrounding diaphragm if different pressures are to exist in the thoracic and peritoneal cavities, yet the defect in the diaphragm (the diaphragmatic hiatus) must be large enough to allow large pieces of food to pass down the esophagus. The seal is formed by a cone of fibrous tissue (phrenoesophageal ligament) which arises from the underside of the diaphragm as an extension of the transversalis fascia and inserts circumferentially around the esophagus (Fig. 1). This allows a pressure gradient to be maintained, yet allows separation between the edges of the diaphragmatic hiatus and the esophagus.
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 Figure 1 |
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| The absence of a serosa allows tumor cells to spread and hampers firm surgical anchoring |
Unlike most of the rest of the gastrointestinal tract, the esophagus is not surrounded by a serosa (covering of mesothelium and subjacent connective tissue). This absence may allow tumor cells to spread unchecked into the mediastinum and may also prevent the surgeon from firmly anchoring anastomotic sutures, thus contributing to the high frequency of leaks after surgery when the esophagus is rejoined to itself or to the stomach.
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2. Blood Supply
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| 3 sources of arterial blood |
The esophagus is supplied with blood from three sources. The upper blood supply to the esophagus comes from the inferior thyroid artery and that to the lower esophagus from a branch of the left gastric artery. The middle of the esophagus is supplied by small branches of the tracheobronchial arteries. There is little overlap in blood supply of the transition zone between the left gastric artery and the bronchial branches so that esophageal anastomosis after a segmental resection requires good surgical technique to avoid ischemia.
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| Esophageal varices |
The esophagus is drained of blood by three main venous pathways: the upper third into the superior vena cava, the middle third into the azygous system, and the lower third via the gastric veins into the portal vein, and then through the liver to return to the heart. This can become of clinical importance if the liver is cirrhotic (scarred by disease) and is impeding portal vein flow; the portal blood from the intestines may flow backward into the gastric veins and enlarge the submucosal veins of the lower esophagus. These enlarged veins (varices) protrude into the lumen of the lower esophagus. Esophageal varices can rupture with dire results.
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3. Innervation
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Nerve supply - Sensory: vagus and sympathetics Motor: mainly vagus |
Sensory impulses are carried in the vagus nerve to the tractus solitarius. It is probable that the thoracic sympathetic chain also carries afferent impulses. The parasympathetic motor supply to the esophagus is derived from the vagus nerve (X) with contributions to the upper portion of the gullet by the glossopharyngeal (IX) and spinal accessory (Xl) nerves. Pain impulses are carried both in the vagal nerve trunk and by fibers which travel from the esophagus to the sympathetic ganglia. There is also an extensive intramural network of nerves which handles both sensory information and modulates motor activity
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4. Histology
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| Esophageal histology |
The esophagus is lined with stratified squamous epithelium (Fig.2). In health, this consists of a basal layer of two to three layers of cells containing dark nuclei and then a relatively thick zone of layers of squamous cells piled one upon another until the lumen is reached. Into this epithelium protrude finger-like pegs of the underlying lamina propria which usually reach less than half the way to the free luminal surface. The lamina propria consists of very loose connective tissue containing lymphocytes and plasma cells which may lie free in the interstitium or be organized into lymphoid nodules. Polymorphonuclear leukocytes are not normally seen. Blood vessels are also evident and often extend up into the pegs. The lamina propria is separated from the underlying submucosa by fibers of the muscularis mucosa, a thin sheet of smooth muscle cells whose function is poorly understood.
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Figure 2 Normal Esophageal Mucosa Legend: BZ – Basal zone SZ – Squamous zone P – Dermal pegs LP – Lamina propria |
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Next Section (D): Esophageal Physiology » |
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