The mediastinum is a central compartment within the chest cavity, nestled between the lungs. It serves as a protected space, housing many vital organs and structures. This region acts as a conduit, allowing various components of the circulatory, respiratory, and digestive systems to pass through the thorax.
Understanding the Mediastinum’s Location
The mediastinum occupies a specific anatomical space. Superiorly, it extends to the thoracic inlet, while inferiorly it reaches the diaphragm, the muscle separating the chest from the abdomen. The sternum, or breastbone, forms its anterior boundary, and the thoracic vertebrae of the spine define its posterior limit. Laterally, the mediastinum is enclosed by the pleural sacs, which encase the lungs on either side.
It is divided into two main parts by an imaginary horizontal line extending from the sternal angle (where the top and main parts of the sternum meet) to the intervertebral disc between the fourth and fifth thoracic vertebrae. This line separates the superior mediastinum from the inferior mediastinum. The inferior mediastinum is further subdivided into three distinct compartments: the anterior, middle, and posterior mediastinum.
Contents of the Superior Mediastinum
The superior mediastinum contains significant vessels, airways, and nerves. The aortic arch, the curved part of the body’s largest artery, originates here, giving rise to major branches such as the brachiocephalic trunk, left common carotid artery, and left subclavian artery, which supply blood to the head, neck, and upper limbs. The superior vena cava, a large vein returning deoxygenated blood to the heart, resides here, formed by the union of the brachiocephalic veins.
The trachea, the main airway, passes through the superior mediastinum before dividing into the main bronchi. The esophagus, a muscular tube for food passage, traverses here, positioned behind the trachea. The vagus and phrenic nerves are found here, playing roles in regulating heart rate, digestion, and diaphragm function. The thymus gland, a lymphatic organ involved in immune system development, is present, though it often atrophies and is replaced by fat in adults.
Contents of the Anterior and Middle Mediastinum
The anterior mediastinum is the smallest subdivision of the inferior mediastinum. It lies directly behind the sternum and in front of the heart’s protective sac, the pericardium. This compartment primarily contains loose connective tissue, some lymph nodes, and remnants of the thymus gland in adults. It serves as a relatively empty space, providing flexibility for the movement of adjacent organs.
The middle mediastinum is considered the most significant part of the inferior mediastinum due to its central contents. It is largely occupied by the heart and its surrounding pericardium. This organ pumps blood throughout the body. The roots of the great vessels that connect directly to the heart, including the ascending aorta, pulmonary trunk, and portions of the superior and inferior vena cava, are also found here.
Additionally, the middle mediastinum houses the bifurcation of the trachea, where it divides into the left and right main bronchi. The phrenic nerves, which control the diaphragm and are essential for breathing, descend through this region, positioned on either side of the heart. Lymph nodes, which are part of the immune system, are also present, particularly around the tracheal bifurcation.
Contents of the Posterior Mediastinum
The posterior mediastinum is located behind the pericardium and diaphragm, extending to the thoracic vertebrae. This elongated space serves as a passageway for structures moving between the chest and abdomen. The descending thoracic aorta, a continuation of the aortic arch, runs through this compartment, supplying blood to the lower body and giving rise to intercostal arteries.
The esophagus, after traversing the superior mediastinum, continues its path through the posterior mediastinum, eventually passing through the diaphragm to reach the stomach. Associated with the esophagus are the vagus nerves, which form plexuses around it, contributing to digestive system control. The azygos and hemiazygos venous systems, which drain blood from the back and chest walls, are also prominent features of this region, ultimately emptying into the superior vena cava.
The thoracic duct, the largest lymphatic vessel in the body, ascends through the posterior mediastinum, collecting lymph from most of the body before returning it to the bloodstream. Sympathetic trunks, which are chains of nerve ganglia involved in the “fight or flight” response, are also situated along the vertebral column.