The mediastinum is the central compartment of the thoracic cavity, situated between the two lungs and their surrounding pleural sacs. This protected region extends vertically from the neck’s base down to the diaphragm, which separates the chest from the abdomen. It serves as a housing unit for major organs and provides a passageway for structures traveling between the head, neck, chest, and abdomen. Key contents include the heart, great vessels, trachea, esophagus, nerves, and lymphatic vessels. Understanding this area requires breaking it down into specific anatomical compartments.
Defining the Boundaries of the Mediastinum
The mediastinum is defined by several boundaries. Anteriorly, it is bounded by the sternum, and posteriorly by the bodies of the thoracic vertebrae. The superior boundary is the thoracic inlet, connecting the chest cavity with the neck, and the inferior limit is the diaphragm muscle. Laterally, the borders are formed by the pleural sacs surrounding the lungs.
Anatomists divide this space using the transverse thoracic plane, an imaginary horizontal line. This line passes from the sternal angle to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4/T5). This division separates the mediastinum into a superior part above the line and an inferior part below it, which is further subdivided into three distinct compartments.
Contents of the Superior Compartment
The superior mediastinum is the area above the T4/T5 plane, acting as a conduit between the neck and the torso. It contains the aortic arch and the three large vessels that branch off it: the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery. The great veins, including the brachiocephalic veins and the upper half of the superior vena cava, are also situated here, returning blood from the upper body to the heart.
Two large tubes occupy this space: the trachea, which carries air, and the esophagus, which transports food. The thymus gland, important for immune function in youth, is positioned anteriorly. Major nerves like the Phrenic and Vagus nerves pass vertically through the superior mediastinum as they travel to reach the diaphragm and abdominal organs.
Contents of the Anterior and Middle Compartments
The inferior mediastinum is divided into the anterior, middle, and posterior compartments. The anterior mediastinum is the smallest subdivision, lying behind the sternum and in front of the pericardium. This narrow space contains loose connective tissue, small lymphatic vessels, and fat, which provides cushioning. It often houses the lower, involuted remnants of the thymus gland, which shrinks after childhood.
Middle Compartment
The middle mediastinum is defined by the contents of the pericardial sac. It is the location of the heart and the roots of the great vessels that attach directly to the heart chambers. These vessels include the ascending aorta, the pulmonary trunk, and the lower segments of the superior and inferior vena cava. This compartment also contains the bifurcation of the trachea into the right and left main bronchi, known as the carina.
The fibrous pericardium, a protective membrane, encloses the heart and anchors it to the diaphragm and the sternum. The phrenic nerves, which control the diaphragm’s movements, descend along the outer surfaces of the pericardium within this compartment.
Contents of the Posterior Compartment
The posterior mediastinum is situated behind the middle compartment and serves as a major pathway for structures descending toward the lower body. The esophagus continues its downward course here, passing behind the heart on its way to the stomach.
A major blood vessel in this area is the descending thoracic aorta, which supplies blood to the chest wall and abdominal organs. Adjacent to the aorta is the thoracic duct, the largest lymphatic vessel, which collects lymph for return to the bloodstream. The azygos and hemiazygos venous systems are also found here, providing drainage for the back and chest walls before emptying into the superior vena cava. Finally, the sympathetic trunks, chains of nerve tissue, run vertically along the sides of the vertebral bodies.