Mediastinum

What Is the Mediastinum?

The mediastinum is the central partition of the thoracic cavity, a thick wall of organs and connective tissue that separates the right and left pleural cavities and their lungs. The name comes from Latin and means roughly the thing standing in the middle.

Mediastinum definition

In anatomy, the mediastinum is defined as the region of the chest between the two pleural sacs, extending from the superior thoracic aperture to the diaphragm and from the sternum to the bodies of the thoracic vertebrae. It contains every organ in the chest except the lungs and their pleura.

Is the mediastinum a cavity?

The mediastinum is often listed as one of the three cavities of the chest, but it is not an empty, fluid-filled space like the pleural cavities. It is a compartment packed with organs, vessels, nerves and fat held together by loose connective tissue. In a living person this tissue is soft and flexible, so the mediastinum can move and change shape with breathing, swallowing and the beating heart.

Explained simply

Picture the chest as a box with two big balloons, the lungs, one on each side. The mediastinum is everything packed between the balloons: the heart in the middle, the pipes that carry blood to and from it, the windpipe and the food pipe running down from the neck, and the wires and drains that go with them.

Where Is the Mediastinum Located?

The mediastinum is located in the center of the chest, behind the breastbone and in front of the spine, filling the space between the medial surfaces of the two lungs. It extends from the base of the neck down to the diaphragm.

• Superior boundary: the superior thoracic aperture, also called the thoracic inlet, where the chest opens into the neck.
• Inferior boundary: the diaphragm, which separates it from the abdomen.
• Anterior boundary: the sternum and the costal cartilages of the rib cage.
• Posterior boundary: the bodies of the 12 thoracic vertebrae.
• Lateral boundaries: the mediastinal part of the parietal pleura on each side, with the lungs beyond it.

Because the mediastinum is open to the neck above, infections or air from the neck can spread down into it. Below, structures pass through openings in the diaphragm to reach the abdomen, as described on the openings of the diaphragm page. At the sides, each lung connects to the mediastinum through its root at the hilum of the lung.

Mediastinum Anatomy: What Are Its Divisions?

The mediastinum is divided into a superior and an inferior mediastinum by an imaginary horizontal plane, the transverse thoracic plane, which runs from the sternal angle at the front to the disc between the fourth and fifth thoracic vertebrae at the back. The inferior mediastinum is then divided into anterior, middle and posterior parts by the pericardium, the sac around the heart.

Superior mediastinum

The superior mediastinum lies above the plane of the sternal angle and below the thoracic inlet. From front to back it contains the thymus, the brachiocephalic veins and upper part of the superior vena cava, the arch of the aorta with its three large branches, the trachea, the esophagus and the thoracic duct. The vagus and phrenic nerves pass through it, and the left recurrent laryngeal nerve hooks under the aortic arch here.

Anterior mediastinum

The anterior mediastinum is the narrow space between the sternum and the front of the pericardium. In adults it holds mainly fat, loose connective tissue, small lymph nodes and branches of the internal thoracic vessels. In children it also contains the lower part of the thymus, which is large in early life and shrinks after puberty.

Middle mediastinum

The middle mediastinum contains the pericardium and the heart, together with the roots of the great vessels: the ascending aorta, the pulmonary trunk and its branches, the lower half of the superior vena cava and the ends of the pulmonary veins. The phrenic nerves run down its sides, and the tracheal bifurcation and main bronchi lie at its upper border.

Posterior mediastinum

The posterior mediastinum lies behind the pericardium and in front of the lower thoracic vertebrae. It contains the descending thoracic aorta, the esophagus, the thoracic duct, the azygos and hemiazygos veins, the vagus nerves forming a plexus on the esophagus, the thoracic splanchnic nerves and posterior lymph nodes.

Compartments used in imaging

Radiologists reading CT scans often use a simpler three-part model instead: a prevascular compartment at the front, a visceral compartment in the middle and a paravertebral compartment beside the spine. It maps the same space in a way that is easier to apply to scan images.

What Is the Location of the Trachea in the Mediastinum?

The trachea lies in the superior mediastinum, in the midline just in front of the esophagus, and it ends at about the level of the sternal angle, where it divides into the right and left main bronchi. Its lower end and the two main bronchi sit at the boundary between the superior and middle mediastinum.

As the trachea descends from the neck through the thoracic inlet, the brachiocephalic veins and the brachiocephalic trunk lie in front of it, and the arch of the aorta curves over its front and left side to reach the back of the chest. The esophagus runs directly behind it, which is why the two tubes are often compared on our trachea vs esophagus page. The point of division is marked inside the airway by a ridge called the carina, roughly at the level of the T4 to T5 vertebrae.

From there the main bronchi run outward to the hila of the lungs, beginning the bronchial tree. The left main bronchus passes under the arch of the aorta and in front of the esophagus and descending aorta, while the right main bronchus passes under the arch of the azygos vein.

What Is the Function of the Mediastinum?

The function of the mediastinum is to house and protect the central organs of the chest, keep the two lungs and their pleural cavities apart, and provide a protected passage for the vessels, airways, nerves and digestive tube that link the neck, lungs and abdomen.

• Holding the heart: the middle mediastinum cradles the heart in its pericardium, close to both lungs, so blood can flow quickly through the pulmonary circulation.
• A passage for air: the trachea and main bronchi travel through it to carry air into the lungs.
• A passage for food: the esophagus runs through the whole length of the mediastinum to the stomach.
• A route for vessels and nerves: the aorta, venae cavae, thoracic duct, vagus nerves and phrenic nerves all pass through it.
• Separating the lungs: because each lung lies in its own pleural sac, a problem on one side, such as a pneumothorax, usually affects only that lung.
• Immune function: the thymus trains T cells in early life, and mediastinal lymph nodes filter lymph from the lungs and heart.

The close packing of the heart, great vessels and airways is what lets the respiratory and circulatory systems work together so efficiently.

Mediastinum vs Pericardial Cavity: What Is the Difference?

The mediastinum is the whole central compartment of the chest, while the pericardial cavity is a thin, fluid-filled space around the heart that lies inside the middle mediastinum. In other words, the pericardial cavity is one small part of the mediastinum.

The heart is wrapped in the pericardium, a sac with a tough outer fibrous layer and an inner serous layer. The serous layer has two sheets, one lining the fibrous sac and one covering the heart itself, and the narrow gap between them is the pericardial cavity. It holds only a thin film of fluid that lets the heart beat with little friction, much as the fluid in the pleural cavity lets the lungs slide.

The difference matters in practice. Fluid collecting inside the pericardial cavity squeezes the heart directly, while air, blood or infection in the rest of the mediastinum spreads through its loose connective tissue around many organs at once.

Common Conditions That Affect the Mediastinum

Conditions of the mediastinum include masses and enlarged lymph nodes, inflammation, air leaking into its tissues and shifts of its position caused by problems in the lungs. Our respiratory diseases section gives more background; anyone with chest symptoms should see a doctor.

• Mediastinal masses: growths are often grouped by compartment. Anterior masses include thymus tumors, germ cell tumors, lymphoma and a thyroid gland extending down into the chest; posterior masses are often nerve tumors.
• Enlarged mediastinal lymph nodes: seen with infections such as tuberculosis, with sarcoidosis and when lung cancer spreads.
• Mediastinitis: infection or inflammation of the mediastinal tissues, for example after a tear in the esophagus or chest surgery.
• Pneumomediastinum: air escaping into the mediastinum, sometimes after a small tear in an alveolus or the airway.
• Mediastinal shift: the mediastinum is pushed or pulled to one side by a large pleural effusion, a tension pneumothorax or collapse of a lung.
• Widened mediastinum: an X-ray finding that can reflect an enlarged aorta, bleeding or a mass.