Thoracic cavity

Summary

The thoracic cavity is a hollow space surrounded by the rib cage and the diaphragm that contains the heart, lungs, esophagus, thymus, sympathetic trunk, and the great vessels. It comprises three compartments: two pleural cavities and the mediastinum, which is located behind the sternum. The mediastinum is divided into the superior and inferior mediastinum, the latter of which is further subdivided into the anterior, middle, and posterior mediastinum. Inflammation of the mediastinum (mediastinitis) can result from spreading retropharyngeal infections or contamination from perforated mediastinal organs. The mediastinum is also a potential site for teratoma, lymphoma, thymoma, and thyroid neoplasm. The pleural cavity is the potential space between the parietal pleura (which covers the thoracic wall and the mediastinum) and the visceral pleura (which lines the lung). The pleura secrets fluid that prevents the development of friction between the two pleural membranes. Excessive build-up of fluid in the pleural cavity results in pleural effusion, while the entry of air results in pneumothorax. The diaphragm is a musculotendinous structure that separates the thoracic cavity from the abdominal cavity. It has hiatuses for the passage of the inferior vena cava (caval hiatus), esophagus (esophageal hiatus), and the aorta (aortic hiatus) at the 8th, 10th, and 12th thoracic vertebrae respectively. The diaphragm is innervated by the phrenic nerve. Phrenic nerve palsy or injury to the diaphragm results in impaired respiration.

Overview

  • 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).

Mediastinum

Overview

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)
Inferior mediastinum Anterior mediastinum
Middle mediastinum
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.

The 4 T's of a mediastinal mass: Thymoma, Teratoma (and other germ cell tumors), Thyroid neoplasm, and Terrible lymphoma!

Vasculature of the mediastinum

Arteries

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

Thoracic aorta

(Part of the 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.

Veins

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

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.

Lymphatics

See thoracic duct and right lymphatic duct

References:[1][2][3][4]

Pleura

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
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).

Pleural fluid is secreted by serous membranes in the pleurae and minimizes friction during the respiratory movements of the lungs. When there is too much fluid in the pleural space, it is termed a pleural effusion.

Diaphragm

Overview

  • 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
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

Anatomy
Arterial supply
Venous drainage
  • Superior and inferior phrenic veins
Innervation
  • 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 03/14/2020
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