5 Esophagus and Thymus and their Neurovasculature
5.1 Esophagus: Location and Divisions
A Projection onto the thoracic skeleton
Anterior view. The esophagus is located slightly to the right of the midline, especially in its course through the thorax, where it descends along the right side of the aorta. It pierces the diaphragm just below the xiphoid process of the sternum. The arrows mark the sites of the three normal anatomical constrictions of the esophagus (see C).
B Divisions of the esophagus
Anterior view with the head turned to the right. The esophagus is approximately 23–27 cm long, 1–2 cm in diameter, and is divided into three parts:
• Cervical part: just anterior to the vertebral column in the neck, extends from C 6 to T 1.
• Thoracic part: the longest part, located in the superior and posterior mediastinum, extends from T 1 to the esophageal hiatus of the diaphragm (at approximately T 11).
• Abdominal part: the shortest part, located in the peritoneal cavity, extends from the diaphragm to the cardiac orifice of the stomach.
C Constrictions and curves of the esophagus
Right lateral view (a), anterior view (b).
The esophagus has three normal anatomical constrictions, which are projected at the levels of specific vertebrae (a). The constrictions are caused by adjacent structures that indent the esophagus and by functional closure mechanisms (lower constriction, see p. 167). These constrictions are visible during gastroscopy, and the scope must be carefully maneuvered past them (normal width of the esophagus is approximately 20 mm):
• Upper constriction (pharyngoesophageal constriction, 14–16 cm from the incisor teeth), corresponds to the esophageal inlet in the cervical part of the esophagus (see p. 166). It is located where the esophagus passes behind the cricoid cartilage (C 6) and has a maximum width of approximately 14 mm.
• Middle constriction (thoracic constriction, 25–27 cm from the incisors), located where the esophagus passes to the right of the aortic arch and thoracic aorta (at T 4/T 5). Maximum width is 14 mm.
• Lower constriction (phrenic constriction, 36–38 cm from the incisors), located at the start of the abdominal part of the esophagus, where it pierces the diaphragm (T 10/T 11). Functional closure of the esophagus by muscles and veins of the esophageal wall. The abdominal part is normally occluded except during swallowing (see p. 167). Maximum width is 14 mm.
Besides its constrictions, the esophagus also presents characteristic curves (b): an upper curve to the left (in the cervical part), a mid-level curve to the right (in the thoracic part, caused by the adjacent thoracic aorta), and a lower curve to the left (in the abdominal part). Additionally, the esophagus is slightly concave anteriorly in the sagittal plane, following the curvature of the vertebral column (thoracic kyphosis, a).
D Topographical relations of the esophagus, posterior view
The relations of the esophagus to the pericardium, great vessels, and trachea are depicted here. The close proximity of the esophagus to the left atrium and thoracic aorta can be seen. Due to the asymmetrical position of the heart in the thorax, the right pulmonary veins are closer to the esophagus than the left pulmonary veins. The esophagus initially descends to the right of the aorta, but just above the diaphragm it crosses in front of the aorta before piercing the diaphragm to enter the abdominal cavity (see C). The esophagus is loosely attached by its own connective tissue (adventitia) to the connective tissue of the mediastinum (important for swallowing). It is stabilized somewhat by the attachment of its anterior wall to the back of the trachea, again by numerous slips of connective tissue.
Note: The trachea develops as an outgrowth from the esophagus during early embryonic development, at which time a communication exists between the two structures. Normally this communication closes, but its persistence results in a tracheoesophageal fistula, which may allow food to enter the trachea and reach the lung, causing recurrent episodes of pneumonia.
5.2 Esophagus: Inlet and Outlet, Opening and Closure
A Esophageal inlet (upper esophageal sphincter)
a Posterior view. The muscular posterior wall of the pharynx has been divided and reflected laterally, and the uppermost esophageal segment has been opened posteriorly. At the posterior junction of the longitudinal esophageal musculature with the pharyngeal musculature, the longitudinal muscles are thin and do not span the full circumference of the esophagus. This area of muscular weakness (“Laimer triangle”) is a site of vulnerability for the development of diverticula (see p. 169). This diagram shows the esophagus with an expanded, stellate lumen near the esophageal inlet, as it would appear during swallowing. While in the resting state, the esophageal inlet usually has the form of a transverse slit. The musculature of the upper esophagus is a continuation of the (skeletal) pharyngeal muscles and consists of striated fibers that give way distally to smooth muscle (not shown here).
b Midsagittal section, viewed from the left side. In the lateral view, both the esophageal muscle and mucosa are visible. Additionally, the diagram shows the posterior dilation of the esophagus, and thus the size of the esophagus relative to the larynx. The upper esophageal constriction, located behind the cricoid cartilage, is also clearly visible.
B Esophageal outlet and esophageal closure
Functional closure of the esophageal outlet is an important mechanism for preventing the backflow of gastric contents, especially hydrochloric acid, into the distal esophagus (gastroesophageal reflux). This mechanism is essential because the esophageal mucosa, unlike the gastric mucosa, is vulnerable to corrosive injury by stomach acid. As a result, repeated exposure to hydrochloric acid can cause esophageal inflammation (reflux esophagitis). Early, relatively mild forms of this reflux (“heartburn”) are often manifested by a burning retrosternal pain that is most pronounced in the supine position (at night). Effective closure of the esophagus is based on several factors:
• Narrowing of the esophageal outlet by
– the circular muscles of the esophagus (see b) and
– submucous venous plexuses, which raise longitudinal folds in the esophageal mucosa (see c). These prominent veins function as portosystemic collaterals in response to an obstruction of portal venous blood flow (see p. 171). Together, the esophageal circular muscles and venous plexuses provide “angiomuscular closure” at the esophagogastric junction;
• The structurally narrow muscular esophageal hiatus in the diaphragm (see c);
• Connective tissue and fat surrounding the esophagogastric junction (c);
• Continuity of the esophageal and gastric musculature (b), and the oblique angle at which the esophagus joins the stomach just below the diaphragm (the angle of His, see a).
5.3 Esophagus: Wall Structure and Weaknesses
A Structure of the esophageal wall
Posterior view. Portions of the pharynx, larynx, and trachea are also shown; and the outermost layer (adventitia, see B) has been removed. The esophageal wall has been telescoped to display both layers of the muscular coat (the circular and longitudinal layers). They are connected to the pharyngeal muscles at the esophageal inlet (hidden here by the pharynx). The muscles of the esophagus can generate powerful peristaltic movements directed toward the stomach (actively propelling a food bolus to the stomach in 5–8 seconds), and they can reverse the direction of these movements during vomiting (antiperistalsis).
B Microscopic structure of the esophageal wall
Transverse section through an esophagus in the contracted (left) and relaxed state (right). The layers of the esophageal wall are typical of a hollow viscus in the digestive tract:
• The mucosa, which consists of an epithelial layer, lamina propria, and muscular layer. The epithelial layer is composed of stratified, nonkeratinized squamous epithelium (for mechanical resistance to food passage).
• The submucosa, a loose layer of connective tissue that contains numerous glands (esophageal glands) whose secretions lubricate the mucosa to facilitate food passage. Particularly in the lower esophagus, the submucosa contains numerous veins that participate in the closure of the esophageal outlet (see p. 167).
• The muscularis, consisting of an inner layer of circular muscle and an outer layer of longitudinal muscle. Smooth-muscle contractions aid in the peristaltic propulsion of food.
• The adventitia, a layer of loose connective tissue that tethers the esophagus to the mediastinal connective tissue and is firmly attached to the connective tissue of the posterior tracheal wall.
