Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
The mediastinum is the central compartment of the thoracic cavity and lies between the two pulmonary cavities.
It’s covered on each side by the mediastinal pleura and contains all thoracic viscera and structures minus the lungs.
The mediastinum is divided into four parts, of which one is superior and three are inferior.
The superior part is bounded superiorly by the thoracic aperture and extends inferiorly to the transverse thoracic plane, which is a horizontal plane that extends from the sternal angle anteriorly and the junction of T4 and T5 posteriorly.
The transverse thoracic plane separates the superior mediastinum from the inferior mediastinum, and is also an important anatomical landmark representing the bifurcation of the trachea and the beginning and end of the arch of the aorta.
The inferior mediastinum lies between the transverse thoracic plane superiorly, and the diaphragm inferiorly.
It’s further subdivided by the pericardium into anterior, middle and posterior parts, with the pericardium and its contents being in the middle part.
So in this video, let’s focus on the superior mediastinum.
From anterior to posterior, it contains the following structures: the thymus, the great vessels, the inferior continuation of the trachea, the inferior continuation of the esophagus, and the thoracic duct and lymphatics trunks.
Now let’s take a closer look at some of these structures one by one.
So, the thymus is located in the inferior part of the neck and in the anterior part of the superior mediastinum.
It is a glandular structure and lies posterior to the manubrium and extends into the anterior mediastinum.
The thymus is a primary lymphoid organ that plays a huge role in helping our immune cells develop before puberty.
After puberty, however, it undergoes gradual involution and is mostly replaced by fat.
Next up, there are the great vessels.
First we have the veins, which include the brachiocephalic veins and the superior vena cava or SVC, as well as the arteries, which include the arch of the aorta and its major branches, the brachiocephalic trunk, left common carotid artery and left subclavian artery.
Let’s take a look at this picture and identify the left and right brachiocephalic veins, which form posterior to the sternoclavicular joints by the joining of the internal jugular and subclavian veins.
Then the brachiocephalic veins join to form the SVC at the level of the inferior border of the 1st right costal cartilage, with the left brachiocephalic vein being longer than the right one, since it has to travel farther to drain to the right side of the heart.
Now, the SVC then passes inferiorly and ends at the level of the 3rd costal cartilage at which point it enters the right atrium.
The SVC lies on the right side of the superior mediastinum, anterolateral to the trachea and posterolateral to the ascending aorta.
The SVC is anterior to the root of the right lung, and the azygous vein drains into its posterior surface.
Next up, there’s the aortic arch, which continues the ascending aorta and begins at the level of the sternal angle, posterior to the 2nd right sternocostal joint.
As you can see in this picture, it arches superiorly, posteriorly and to the left and then inferiorly.
The arch goes up anterior to the right pulmonary artery and the bifurcation of the trachea.
It reaches its apex as it passes over the root of the left lung, at the left side of the trachea and esophagus.
The arch descends posterior to the left root of the lung beside T4 and ends by becoming the thoracic or descending aorta posterior to the 2nd left sternocostal joint.
Now for a really quick break - can you recall the plane that the beginning and end of the arch of the aorta is divided by? That's right!
We’re talking about the transverse thoracic plane!
It arises posterior to the manubrium, at which point is anterior to the trachea and posterior to the left brachiocephalic vein.
Then, it ascends superiorly and laterally to reach the side of the trachea and is found posterior to the right sternocostal joint, where it divides into the right common carotid and right subclavian artery.
The left common carotid artery is the second branch of the aortic arch.
It originates posterior to the manubrium, but slightly posterior and to the left of the brachiocephalic trunk.
It ascends anterior to the left subclavian artery and enters the neck posterior to the left sternocostal joint.
It ascends through the superior mediastinum lateral to the left common carotid artery and It enters the root of the neck by passing posterior to the left sternocostal joint.
One last important structure in the superior mediastinum to mention is the ligamentum arteriosum.
The ligamentum arteriosum is a fibrous band, which is an embryological remnant of the ductus arteriosus from birth, which acted as a shunt for blood to travel from the pulmonary trunk to the aortic arch in order to bypass the fetal lungs, which weren’t functional in utero.
The ligamentum arteriosum remains and passes from the superior portion of the proximal left pulmonary artery to the inferior surface of the aortic arch.