Chapter 21: The esophagus, trachea and main bronchi
The esophagus (figs. 21-1, 21-2 and 21-3) has cervical, thoracic, and abdominal parts, extending from the lower end of the pharynx (C6 vertebrae) to the cardiac opening of the stomach (T11 or 12 vertebral level). When a subject is in the erect position, it is about 25 to 30 cm long. The esophagus is a median structure that lies first behind the trachea and then behind the left atrium. It begins to deviate to the left below the left main bronchus. In the posterior mediastinum it is related to the vertebral column as a string is related to a bow. Hence there is a (retrocardiac) space between it and the vertebrae, which is visible radiographically in oblique and lateral views.
The esophagus has constrictions (1) at its commencement, (2) frequently where it is crossed by the left main bronchus, and (3) commonly where it traverses the diaphragm. These are sites where swallowed objects can lodge notwithstanding the fact that the esophagus is distensible and can accommodate almost anything that can be swallowed, e.g., a denture. The impressions of adjacent structures, and their alterations in disease, can be seen radiographically after barium is swallowed (fig. 21-3).
The esophagus transports food and liquid and can be replaced successfully by a non-muscular tube. The muscular layer is striated above (supplied by the vagi) and smooth below (supplied by parasympathetic, or vagal, and sympathetic fibers).
The process of swallowing may be watched fluoroscopically. A thin barium meal or liquid is "shot down" to the cardiac orifice, whereas a thick meal or a bolus of food travels more slowly.
The esophagus is supplied by arteries in the neck (inferior thyroid arteries), thorax (bronchial arteries, direct branches of the aorta, and phrenic arteries), and abdomen (left gastric artery). Veins of the lower part of the esophagus communicate with the left gastric vein, thereby forming an important portal-systemic anastomosis. Portal obstruction (e.g., in the liver with cirrhosis) causes these channels to enlarge, and their varicosities may produce hemorrhage.
Pain fibers from the esophagus accompany the sympathetic system. A vague, deep-seated, esophageal pain may be felt behind the sternum or in the epigastrium, and it resembles that arising from the stomach or heart ("heartburn").
In esophagoscopy, measurements are taken from the upper incisor teeth to indicate the beginning of the esophagus (18 cm), the point at which it is crossed by the left bronchus (28 cm), and its termination (43 cm).
The trachea, or windpipe (figs. 21-1 and 21-2), which has cervical and thoracic parts, extends from the inferior end of the larynx (C6 vertebra) to its point of bifurcation (between T5 and 7 vertebral level). It is about 9 to 15 cm in length. The trachea descends anterior to the esophagus, enters the superior mediastinum, and divides into right and left main bronchi. The trachea is a median structure but, near its lower end, deviates slightly to the right, resulting in the left main bronchus crossing anterior to the esophagus. Owing to the translucency of the air within it, the trachea is usually visible above the arch of the aorta in radiographs.
The trachea has 15 to 20 C-shaped bars of hyaline cartilage that prevent it from collapsing. Longitudinal elastic fibers enable the trachea to stretch and descend with the roots of the lungs during inspiration. When a subject is in the erect position, the trachea divides between the T5 and T7 vertebral levels. The carina is the upward-directed ridge seen internally at the bifurcation and is a landmark during bronchoscopy.
The arch of the aorta is at first anterior to the trachea and then on its left side immediately superior to the left main bronchus. Other close relations include the brachiocephalic and left common carotid arteries. The trachea is supplied mainly by the inferior thyroid arteries. Its smooth muscle is supplied by parasympathetic and sympathetic fibers, and pain fibers are carried by the vagi.
Each main bronchus extends from the tracheal bifurcation to the hilus of the' corresponding lung. The right main bronchus may be considered as having (1) an upper (eparterial) part, from which the segmental bronchi for the upper lobe arise, and (2) a lower part, from which the segmental bronchi for the middle and lower lobes emerge (fig. 21-4). The left main bronchus divides into two lobar bronchi, one each for the upper and lower lobes. The upper lobar bronchus may be considered as having (1) an upper division and (2) a lower, or lingular, division.
The right main bronchus, about 2.5 cm in length, is shorter, wider, and more nearly vertical than the left. Because it is in almost a direct line with the trachea, foreign objects traversing the trachea are more likely to enter the right main bronchus. The left main bronchus, 5 cm or more in length, crosses anterior to the esophagus (fig. 21-2), which it indents. Both bronchi have cartilaginous rings that are replaced by separated plates at the roots of the lungs. The bronchi are supplied by the bronchial arteries and veins, and their innervation is similar to that of the trachea.
Terracol, J., and Sweet, R. H., Diseases of the Esophagus, W. B. Saunders Company, Philadelphia, 1958. A general reference with an extensive bibliography.
21-1 What lies anterior to the esophagus in the lower part of the thorax?
21-2 Where does the trachea divide?
21-3 How does the right main bronchus differ from the left?
Figure 21-1 The trachea and esophagus in relation to vertebral and sternal levels in a subject in the erect position.
Figure 21-2 The relations of the trachea, bronchi, esophagus, and aorta to one another. In the right anterior oblique view, the right lobar and segmental bronchi are omitted because they are not clearly visible in radiographs of this view. For similar reasons, the left lobar and segmental bronchi are omitted from the left anterior oblique view. The horizontal line indicates the level of the carina.
Figure 21-3 Oblique (R.A.O.) view of thorax. The esophagus is shown coated with barium. Note the vertical folds of the mucosa and the indentations produced by the arch of the aorta (upper white arrow) and by the left main bronchus (lower white arrow). Note also the right and left domes of the diaphragm (on the left and right sides of the illustration, respectively) and the fluid level in the stomach (black arrow). (From Bassett, D.L., A Stereoscopic Atlas of Human Anatomy, Sawyer's, Portland, Oregon, 1958. Copyright 1958, Sawyer's Inc.)