Lungs

Mediastinum

The mediastinum is a compartment located between two pleural sacs.

The borders are:

  • Anterior: Sternum.
  • Posterior: Vertebral column (bodies of thoracic vertebrae and intervening intervertebral discs T1-T12).
  • Superior: Superior thoracic aperture.
  • Inferior: Diaphragm.
  • On every side: Mediastinal pleura
Mediastinum

The mediastinum is divided into two main parts by an imaginary line that runs from the sternal angle to the T4 vertebrae:

  1. Superior Mediastinum – extends upwards, terminating at the superior thoracic aperture.
  2. Inferior Mediastinum – extends downwards, terminating at the diaphragm. The inferior mediastinum is further divided into anterior, middle and posterior. 

Superior Mediastinum

Borders

  • Superior – Superior thoracic aperture. An oblique plane extending from the jugular notch of the manubrium to the superior border of T1 vertebra.
  • Inferior – Continuous with the inferior mediastinum, a transverse plane extending from the sternal angle to the T4-T5 intervertebral disc.
  • Anterior – Manubrium of the sternum.
  • Posterior – Vertebral bodies of T1-4.
  • Lateral – Mediastinal pleura

Contents

  • Great Vessels
      1. Aortic arch including all three main branches – brachiocephalic, left subclavian, left common carotid;
      2. Tributaries of SVC – brachiocephalic veins, left superior intercostal vein, supreme intercostal vein and azygos vein.
  • Nerves
      1. Vagus nerve – right and left
      2. Phrenic nerve 
      3. Sympathetic trunk
      4. Cardiac nerves
  • Other structures
      1. Thymus
      2. Trachea
      3. Thoracic Duct
      4. Oesophagus

Inferior Mediastinum

Anterior Mediastinum

Borders

  • Superior – Continuous with the superior mediastinum at the level of the sternal angle, T4
  • Inferior – Diaphragm.
  • Anterior – Body of the sternum and the transversus thoracis muscles.
  • Posterior – Anterior borders of pericardium
  • Lateral – Mediastinal pleura

Content (this is the largest division and contains important structures)

  1. Inferior portion of thymus (in infants and children)
  2. Fat
  3. Connective tissue
  4. Lymph nodes
  5. Mediastinal branch of internal thoracic vessels
  6. Sternopericardial ligaments

Middle Mediastinum

Borders

  • Superior – Continuous with the superior mediastinum at the level of the sternal angle, T4
  • Inferior – Diaphragm.
  • Anterior – Anterior border of pericardium
  • Posterior – Posterior border of pericardium
  • Lateral – Mediastinal pleura

Contents

  • Great Vessels
      1. Ascending aorta
      2. SVC
      3. Pulmonary trunk – branching into right and left pulmonary arteries
  • Nerves
      1. Cardiac plexus – The sympathetic nerves are derived from T1-T4 spinal cord segments, and the vagus nerve provide parasympathetic innervation.
      2. Phrenic nerve – left and right 
  • Other structures
      1. Heart
      2. Pericardium
      3. Tracheal bifurcation – right and left main bronchus

Posterior Mediastinum

Borders

  • Superior – Continuous with the superior mediastinum at the level of the sternal angle, T4
  • Inferior – Diaphragm.
  • Anterior – Posterior border of the pericardium
  • Posterior – T5-T12 vertebrae.
  • Lateral – Mediastinal pleura

Contents

  1. Thoracic aorta including – posterior intercostal, bronchial, oesophageal, and superior phrenic arteries.
  2. Azygous system of veins
  3. Thoracic duct
  4. Oesophagus
  5. Oesophageal plexus
  6. Sympathetic trunks
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