Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
As we know, the mediastinum is divided into the superior mediastinum and the inferior mediastinum.
The superior mediastinum is separated from the inferior mediastinum by the transverse thoracic plane, which is a horizontal plane that extends from the sternal angle anteriorly and the junction of T4 and T5 posteriorly, an important anatomical landmark representing the bifurcation of the trachea and the beginning and end of the arch of the aorta.
So first, let’s look at the posterior mediastinum.
The posterior mediastinum contains the thoracic aorta, thoracic duct and lymphatic trunks, posterior mediastinal lymph nodes, azygos and hemiazygos veins, the esophagus and the esophageal nerve plexus.
The thoracic aorta continues the aortic arch and it begins on the left side of the inferior border of the body of T4, moving downwards in the posterior mediastinum on the left side of T5 through T12 vertebrae.
On its way down, the thoracic aorta is close to the median plane and moves the esophagus towards the right.
The thoracic aorta lies posterior to the root of the left lung, pericardium and esophagus and is surrounded by the thoracic aortic plexus.
The thoracic duct and azygos vein climb on the right side of the thoracic aorta and accompany it through the aortic hiatus.
The vascular branches of the thoracic aorta are: the esophageal arteries, which supply the esophagus, the bronchial arteries which supply the lungs, the 9 pairs of posterior intercostal arteries that supply all the intercostal spaces except for the upper two, and the subcostal arteries which supply the muscles and skin below the 12th rib.
Other branches include the superior phrenic arteries, which supply part of the diaphragm and pericardial branches, which help supply the pericardium.
Now, the posterior mediastinum also contains the inferior portion of the esophagus as it continues its course from the superior mediastinum.
In the posterior mediastinum, the esophagus passes posterior and to the right of the aortic arch and posterior to the pericardium and left atrium, so it’s the main structure posterior to the base of the heart.
Then, it deviates to the left and passes through the esophageal hiatus in the diaphragm at the level of T10, and at this point it’s anterior to the aorta.
Remember that the thoracic aorta passes through the diaphragm at the level of T12, and the esophagus passes at level T10.
It’s also important that the inferior vena cava ascends through the diaphragm at level T8.
To remember these anatomical landmarks, think of the phrase “I ate 10 eggs at 12”.
The ‘I’ represent the IVC at ’ate’, the eggs represent the esophagus at T10, and the at 12 is aorta at T12!
Now, the esophagus may have 3 anatomic impressions or constrictions in the posterior mediastinum caused by three structures it meets on its way down: the aortic arch, the left main bronchus and the diaphragm.
These narrowings can be seen in oblique chest radiographs if a person were to undergo a barium swallow study.
Also know that there is another potential constriction of the proximal esophagus prior to it entering the posterior mediastinum at the level of the cricoid cartilage.
Remember that the surface esophagus is covered by the esophageal plexus, which is derived from the left and right vagus nerves as they descend through the superior and posterior mediastinum.
The esophageal plexus goes on to form the anterior and posterior vagal trunk found on the inferior esophagus just before the esophagus passes through the diaphragm.
These vagal trunks pass through the diaphragm with the esophagus and go on to innervate a large part of the GI tract.
Let’s now tackle the veins of the azygos venous system.
So, the azygos vein travels proximally up the posterior mediastinum and forms a collateral pathway from the posterior aspect of the inferior vena cava at the level of the renal veins.
In the posterior mediastinum, the azygos vein passes close to the right side of the bodies of the inferior 8 thoracic vertebrae posterior to the root of the right lung.
Then it arches over the superior aspect of the root of the right lung to join the superior vena cava.
The azygos vein also receives the mediastinal, esophageal and bronchial veins.
Now, the hemiazygos vein originates on the left side of the vertebral column, posterior to the thoracic aorta as far as the T9 vertebra.
At this point, it crosses to the right, posterior to the aorta, thoracic duct and esophagus and joins the azygos vein.
It receives the inferior three posterior intercostal veins, the inferior esophageal veins and many small mediastinal veins.
The accessory hemiazygos vein originates at the medial end of the 4th or 5th intercostal space and goes down on the left side of the vertebral column from T5 through T8.
It receives blood from veins in the 4th through 8th intercostal spaces and sometimes even from left bronchial veins.
It crosses over the T7 or T8 vertebra, posterior to the thoracic aorta and thoracic duct, at which point it joins the azygos vein.
Okay, let’s now talk about the thoracic duct, which is the largest lymphatic channel in the body and conveys most of the lymph in the body to the venous system, specifically all lymph except from the right superior quadrant of the body.
In the posterior mediastinum, it lies on the anterior part of the bodies of T12 to T5.
The thoracic duct originates from the cisterna chyli in the abdomen and enters the mediastinum through the aortic hiatus in the diaphragm at the level of T12.
It then goes up in the posterior mediastinum among the thoracic aorta on its left, the azygos vein on its right, the esophagus anteriorly and the vertebral bodies posteriorly.
At the level of T4, T5 or T6, the thoracic duct crosses to the left, becoming posterior to the esophagus and enters the superior mediastinum.
The thoracic duct typically empties into the venous system close to the joining of the left internal jugular and subclavian veins at a point called the left venous angle, or origin of the brachiocephalic vein.