Thoracic cavity


  • The thoracic cavity:
    • Is demarcated by the rib cage, and it is separated from the abdominal cavity by the diaphragm.
    • Is primarily divided into three cavities:
      • The centrally located mediastinum
      • Two laterally located pleural cavities
    • Contains the heart, lungs, esophagus, thymus, the sympathetic trunk, and the great vessels (e.g., aorta, thoracic trunk).



Compartments of the mediastinum

The mediastinum is divided into four different compartments.

Structure Boundaries Content Causes of mediastinal masses
Superior mediastinum
  • Superior: first rib
  • Inferior: line that courses from the sternal angle to the 4th and 5th vertebral levels (transthoracic plane)
  • Thymomas
  • Lymphomas
Inferior mediastinum Anterior mediastinum
Middle mediastinum
  • Anterior and posterior pericardium
  • Between the two pleural cavities
Posterior mediastinum

The most common causes of inflammation of the mediastinal space (= mediastinitis) include spreading of retropharyngeal infections into the mediastinum, chest trauma, or perforation of mediastinal organs, e.g., the esophagus in the course of a gastroscopy or due to esophageal cancer. Symptoms include fever, retrosternal and/or back pain, and subcutaneous emphysema in the neck and face. Acute mediastinitis can be life-threatening and must be treated with surgical debridement, IV antibiotics, and treatment of the underlying cause.

Vasculature of the mediastinum


Structure Anatomy Location Branches Supplies
Ascending aorta
Arch of the aorta
  • Head
  • Upper extremities

Thoracic aorta

(Descending aorta)

The pulmonary trunk, which is located in the middle mediastinum and conveys deoxygenated blood to the lungs, is discussed in airways and lungs.


Superior vena cava (SVC)

Brachiocephalic veins

Brachiocephalic vein obstruction

  • Etiology: thrombosis (e.g., due to central vein catheter), stenosis, external compression, or vascular tumor invasion (e.g., due to apical lung tumor, lymphoma, thymoma, seminoma)
  • Clinical features
    • Unilateral upper extremity swelling, pain, and weakness
    • Unilateral facial and neck swelling, venous dilatation
  • Treatment
    • Balloon angioplasty with or without stenting
    • Surgical bypass in resistant cases

Azygos venous system

Structure Characteristics Tributaries

Azygos vein

Hemiazygos vein

  • Courses along the left side of the vertebral column and passes behind the aorta
  • Unpaired vein that drains into the azygos vein
  • Formed by the ascending lumbar and left subcostal veins
  • Enters the thorax via aortic hiatus or the left crus of the diaphragm
  • Connected to the left renal vein inferiorly

Accessory hemiazygos vein

  • Unpaired vein that usually drains into the azygos vein at the level T8
  • Formed by the 4th to 8th left posterior intercostal veins
  • Located on the left side of the vertebral column
  • 4th–8th left posterior intercostal veins

The pulmonary veins, which are located in the middle mediastinum and transfer oxygenated blood from the lungs to the heart, are discussed in airways and lungs.

The azygos vein connects the inferior and superior vena cava and can be an alternative pathway for deoxygenated blood to return to the right atrium if the SVC or the IVC are obstructed.


See thoracic duct and right lymphatic duct


The pleura is a thin double layer of tissue that surrounds the lungs and the thoracic wall. The pleura is divided into a visceral and parietal layer. The potential space between the parietal and visceral pleura is referred to as the pleural cavity or pleural sac. The visceral pleura is closely adherent to the parenchyma of the lungs, whereas the parietal pleura; is adherent to the thoracic wall. In addition, the parietal pleura is intimately related to the endothoracic fascia, a thin fibroareolar layer of tissue that lines the thoracic cavity.

Structure Characteristics
Pleural cavity
Parietal pleura
  • Lines the inner surface of the thoracic wall and the mediastinum
  • Divided into cervical, mediastinal, costal, and diaphragmatic parts
    • The cervical part projects above the first cervical rib into the neck
  • Separated from the thoracic wall by the endothoracic fascia
  • Innervation
    • Costal part: intercostal nerve (very sensitive to pain)
      • Rib fractures, pleuritis, and pleural masses (e.g., mesotheliomas) cause sharp chest pain that is worse with inhalation (pleuritic).
    • Diaphragmatic part: phrenic nerve
  • Blood supply
    • Internal thoracic artery
    • Posterior and superior intercostal arteries
    • Superior phrenic artery
Visceral pleura
  • Lines the outer surface of the lungs
  • Follows the contour of the lungs, including the fissures
  • Innervation: bronchial plexus (not sensitive to pain, temperature, or touch)
    • Not sensitive to pain → intrapulmonary pathologies (e.g., lung cancer) often remain unnoticed for a long time
  • Blood supply: bronchial arteries

If air enters the pleural cavity, the surface tension between the parietal and visceral pleura of the lungs is lost, letting the lungs collapse due to their natural tendency to collapse or recoil (pneumothorax).



  • Structure
    • C-shaped musculotendinous structure that separates the thoracic cavity from the abdominal cavity
    • Forms a dome between the two cavities, with the superior surface making the floor of the thoracic cavity, and the inferior surface making the roof of the abdominal cavity
    • Anatomically, the diaphragm is divided into a right and left hemidiaphragm; however, the diaphragm is a continuous structure.
      • The right hemidiaphragm is higher than the left due to the presence of the liver.
  • Function
    • Separates the thoracic cavity from the abdominal cavity
    • Key muscle of inspiration

Parts of the diaphragm

Structures Characteristics
Crura of the diaphragm
  • Musculotendinous bundles that attach to the anterior surface of the vertebral column, intervertebral disc, and the anterior longitudinal ligament
  • Origin: lumbar vertebrae (L1, L2, L3) and medial and lateral arcuate ligaments (on the sides)
  • Insertion: central tendon
  • Right crus: longer
  • Left crus: shorter
  • The right and left crura joint together via the median arcuate ligament
  • The diaphragm also attaches to the medial and lateral arcuate ligaments
    • Fascial thickenings of the quadratus lumborum and psoas muscles
Costal diaphragm
Central tendon of the diaphragm
  • Point of convergence of the peripherally located muscles that make up the diaphragm
Openings of the diaphragm

Vasculature of the diaphragm

Arterial supply
Venous drainage
  • Superior and inferior phrenic veins
  • Phrenic nerve (C3–C5)
  • Inferior intercostal nerves and subcostal nerves (sensory innervation to peripheral parts of the diaphragm)

C3,4, 5 keeps the diaphragm alive!

Injuries to the diaphragm or phrenic nerve palsies result in respiratory abnormalities because the diaphragm is the primary muscle of respiration.

Structures passing through the diaphragm

Opening Level Structure
Caval hiatus
  • T8
Esophageal hiatus
  • T10
Aortic hiatus
  • T12

The protrusion of an abdominal structure/organ into the thorax through a lax diaphragmatic esophageal hiatus is referred to as a hiatal hernia.

Clinical significance

last updated 09/01/2019
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