Mediastinum & external heart features

Definition & Location

• The mediastinum is the central compartment of the thoracic cavity.
• Located between the two lungs (between the pleurae).
• Contains the heart, great vessels, trachea, esophagus, thymus, nerves, and lymphatics.
• Divides the thorax into right and left pleural cavities.

Boundaries of the Mediastinum

• Anterior: Sternum.
• Posterior: Thoracic vertebrae (T1–T12).
• Lateral: Pleura of the lungs.
• Superior: Thoracic inlet.
• Inferior: Diaphragm.

Divisions of the Mediastinum

1. Superior mediastinum (above T4)
2. Inferior mediastinum (below T4) → subdivided into:
3. • Anterior
   • Middle
   • Posterior

Plane of Division

• Transverse thoracic plane:
• • Anteriorly: Sternal angle (Angle of Louis)
  • Posteriorly: Disc between T4 and T5

Superior Mediastinum (Above T4)
Main contents: Great vessels, nerves, airways, lymphatics.

Vessels:

• Arteries: Aortic arch and upper pulmonary trunk.
• Veins: Superior vena cava, brachiocephalic veins, and upper azygos vein.

Nerves:

• Vagus nerve.
• Phrenic nerve.
• Cardiac nerves.

Other structures:

• Trachea.
• Esophagus.
• Thoracic duct (ascending).
• Thymus (upper part).
• Lymph nodes and vessels.

Inferior Mediastinum (Below T4)

A. Anterior Mediastinum

• Location: Between sternum and pericardium.
• Contents: Lower part of thymus, lymph nodes and small vessels, loose connective tissue, and fat.

B. Middle Mediastinum

• Main content: The heart within the pericardial sac.
• Contents:
• • Heart.
  • Ascending aorta.
  • Lower pulmonary trunk.
  • Pulmonary veins (4 entering the left atrium).
  • Phrenic and vagus nerves (on both sides).
  • Tracheal bifurcation (into main bronchi).

C. Posterior Mediastinum

• Location: Behind the pericardium.
• Contents:
• • Esophagus.
  • Descending aorta.
  • Azygos and hemiazygos veins.
  • Thoracic duct.
  • Vagus nerves.
  • Sympathetic trunk.

Clinical Importance

• Mediastinal masses may compress the heart, vessels, trachea, or esophagus.
• Mediastinitis: inflammation or infection (after surgery or trauma).
• Mediastinal shift occurs in pneumothorax or pleural effusion.
• Thymoma: tumor of the thymus (often in the anterior mediastinum).