RELATIONSHIPS OF THE TRACHEA AND MAIN BRONCHI
The trachea begins at the lower border of the larynx (just below the cricoid cartilage) at about the level of the sixth cervical vertebra and ends at about the level of the upper border of the ﬁfth thoracic vertebra, where it divides into the two main bronchi. The thyroid gland lies on the anterior and both lateral aspects of the highest part of the trachea.
As the aorta arches over the root of the left lung, it ﬁrst lies anterior to the trachea and then on its left side. The major arteries arising from the aortic arch are in close relationship with the trachea. The brachiocephalic (innominate) artery at ﬁrst is anterior to the trachea and then is on its right side before dividing into the right common carotid and right subclavian arteries. The left common carotid artery is ﬁrst anterior to and then on the left lateral aspect of the trachea.
The left brachiocephalic (innominate) vein crosses from left to right, anterior to the trachea and partly separated from it by the major branches of the aortic arch. The right brachiocephalic vein is separated from the trachea by the right brachiocephalic artery.
The beginning of the right main bronchus lies ante- rior to the esophagus. As it courses inferiorly and laterally to divide into the lobar bronchi, it is posterior to the right pulmonary artery. The bronchus crosses in front of the azygos vein and is separated from the thoracic duct by the esophagus. The relationship to other structures at the root of the lung is shown in Plate 1-13. The beginning of the left main bronchus also lies anterior to the esophagus, from which it runs laterally and inferiorly to reach the hilum of the left lung. Because its course is less vertical than that of the right main bronchus (less in a direct line with the trachea), foreign bodies are a little more likely to enter the right bronchus than the left.
The left recurrent laryngeal nerve arises from the left vagus nerve as it crosses the arch of the aorta and swings posteriorly to loop around the aortic arch just lateral to the ligamentum arteriosum. This nerve then runs cranially in the groove between the trachea and the esophagus to reach the larynx.
The esophagus starts as a continuation of the pharynx at the lower border of the larynx and continues through the thorax. It then passes through the esophageal aperture of the diaphragm to enter the abdominal cavity and terminate at the stomach.
The ligamentum arteriosum, the remnant of the ductus arteriosus, runs from the beginning of the left pulmonary artery to the undersurface of the arch of the aorta. In fetal life, the ligamentum arteriosum shunts blood from the pulmonary artery to the aorta, so that fetal blood does not pass through the pulmonary circulation.
The vagus nerves split into several bundles below the root of the lung and form the esophageal plexus on the surface of the esophagus. Other contributions to the plexus come from the sympathetic trunks and splanchnic nerves. At the lower end of the plexus, two trunks are formed, which pass through the esophageal aperture of the diaphragm. The anterior trunk is mostly derived from the left vagus and the posterior trunk mostly from the right vagus.
Also worthy of note are the pulmonary veins, shown cut at the roots of the right and left lungs; the parietal pleura, cut to expose the lungs, each of which is covered by visceral pleura; the cut edge of the pericardium; and the inferior vena cava passing through the diaphragm.