The mediastinum is the central compartment situated within the chest cavity (thorax). This region is nestled between the two lungs and serves as a protective area for many of the body’s most important organs. It provides a protected pathway for structures passing between the neck, the abdomen, and the limbs.
Location and Defining Boundaries
The mediastinum’s location is defined by four distinct boundaries. Laterally, the compartment is bordered by the pleural sacs, which separate it from the lungs. The anterior boundary is the sternum (breastbone). The posterior boundary is the vertebral column, providing bony support at the back of the space. The inferior border is the diaphragm, the muscle separating the thoracic and abdominal cavities.
Understanding the Key Divisions
Anatomists divide the mediastinum into a superior and an inferior section to simplify the study of its complex structures. This primary division is marked by the transverse thoracic plane, an imaginary horizontal line. This plane extends from the sternal angle backward to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4/T5).
The Superior Mediastinum lies above this line, reaching up to the thoracic inlet. The Inferior Mediastinum is the larger area below the plane, extending down to the diaphragm. This inferior section is further compartmentalized into three distinct regions defined by the pericardium, the sac surrounding the heart.
The Anterior Mediastinum is the smallest subdivision, located in front of the pericardium and behind the sternum. The Middle Mediastinum is the central portion of the inferior section, defined by the pericardium and its contents. The Posterior Mediastinum is the space situated behind the pericardium and the heart, extending back to the thoracic vertebral column.
Essential Structures Housed Within
The Middle Mediastinum is the most substantial part, containing the heart enclosed within its protective pericardial sac. The roots of the great vessels, such as the ascending aorta and the pulmonary trunk, also originate in this central space.
The Posterior Mediastinum acts as a vertical corridor for structures passing toward the abdomen. The esophagus, the muscular tube that carries food, descends through this area just behind the heart. Prominent contents also include the descending thoracic aorta, the major artery carrying blood to the lower body, the thoracic duct (the largest lymphatic vessel), and the sympathetic nerves.
The Superior Mediastinum houses structures passing from the neck into the thorax, including the trachea (windpipe) before it branches into the lungs. Major blood vessels, such as the arch of the aorta and the superior vena cava, run through this upper space, along with the phrenic and vagus nerves.
In children, the thymus gland, important for immune development, occupies a large part of the superior and anterior mediastinum. In adults, the Anterior Mediastinum is mainly filled with loose connective tissue, fat, lymph nodes, and remnants of the involuted thymus.
Clinical Significance
The distinct anatomical divisions of the mediastinum are fundamental for diagnosing and treating various medical conditions. Knowledge of these compartments helps clinicians determine the likely source of a mass or tumor found on a chest scan. For instance, a mass in the Anterior Mediastinum is frequently a thymoma (a tumor of the thymus) or a lymphoma. Growths in the Posterior Mediastinum are often neurogenic tumors arising from nerve tissue.
A primary clinical finding is mediastinal widening, which is the enlargement of the central chest shadow seen on a chest X-ray. This finding can signal several serious underlying issues due to the dense packing of structures in the area. Potential causes include a rupture or dissection of the aorta, a life-threatening emergency, or severe infection of the space known as mediastinitis. Widening can also result from chronic conditions like a large mass or extensive lymph node enlargement.