Thoracic cavity

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 secretes 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 8^th, 10^th, and 12^th thoracic vertebrae respectively. The diaphragm is innervated by the phrenic nerve. Phrenic nerve palsy or injury to the diaphragm results in impaired respiration.

• Boundaries: demarcated by the rib cage, and separated from the abdominal cavity by the diaphragm ^[1]
• Divisions
  • The centrally located mediastinum
  • Two laterally located pleural cavities
• Contents
  • Organs: heart, lungs, esophagus, thymus
  • Nerves: sympathetic trunk
  • Vessels (e.g., aorta, thoracic trunk)

Overview

• Definition: The mediastinum is the central space of the thoracic cavity located behind the sternum and between the two lungs and their respective pleura. ^[1]
• Boundaries
  • Anterior: sternum and transversus thoracis muscles
  • Posterior: vertebral column
  • Superior: thoracic inlet
  • Inferior: diaphragm
  • Lateral: pleura of the lung

The most common causes of inflammation of the mediastinal space (mediastinitis) include spreading of retropharyngeal infections into the mediastinum, chest trauma, and perforation of mediastinal organs, e.g., the esophagus in the course of a gastroscopy or due to esophageal cancer.

The trachea bifourcates at the level of T4.

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

Vasculature of the mediastinum ^[1]

Arteries

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

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”. Cavocaval anastomoses are discussed in “Abdominal cavity”.

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.”

Overview ^[1]

Definition: Pleura is a thin double layer of tissue that surrounds the lungs and the thoracic wall and divided into a visceral and parietal layer.

Pleural cavity ^[1]

• Location: potential space between the parietal and visceral pleura of the lungs
• Content: pleural fluid (secreted by serous membranes in the pleura)
  • Lubricates and prevents friction between the parietal and visceral pleura
  • Produces a surface tension that draws the two pleura layers together, ensuring simultaneous extension of the thorax and lungs during respiration
• Recesses
  • Costomediastinal recess: region where the mediastinal and costal pleura meet
  • Costodiaphragmatic recess: region where the costal and diaphragmatic pleura meet
    • Provides potential space for the lungs to expand during inspiration and movement of the diaphragm
    • Potential space for air, blood, and/or fluid accumulation (e.g., pleural effusion, pneumothorax, hemothorax)

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).

Excessive fluid in the pleural cavity results in pleural effusion.

Overview ^[1]

• 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 ^[1]

Crura of the diaphragm

• Definition: 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
• Structure
  • Right crus: longer
  • Left crus: shorter
  • The right and left crura join together via the median arcuate ligament.
  • The diaphragm also attaches to the medial and lateral arcuate ligaments

Costal diaphragm

• Origin: lower six ribs and xiphoid process
• Insertion: central tendon

Central tendon of the diaphragm

• Location: point of convergence of the peripherally located muscles that make up the diaphragm

Structures passing through the diaphragm

I ate (read 8) ten Eggs At twelve: Inferior vena cava passes the diaphragm at T8, the Esophagus at T10, and the Aorta at T12.

The vagus nerve (CN X) passes through the diaphragm at T10.

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

Vasculature and innervation of the diaphragm

C3, C4, C5 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.

Mediastinum

Brachiocephalic vein obstruction ^[4][5]

• 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

Others

• Mediastinitis
• Pneumomediastinum
• Retrosternal thyroid (substernal goiter)
• Bilateral hilar lymphadenopathy
• Mediastinal tumors
  • Thymoma (most common primary anterior mediastinal tumor)
  • Lymphomas (Hodgkin lymphoma, non-Hodgkin lymphoma)
  • Pheochromocytoma
  • Extragonadal germ cell tumors (e.g., teratoma)
• Mediastinal vessels
  • Aortic dissection
  • Aortic rupture
  • Aortitis
  • Aortic aneurysm
  • Aortic coarctation
  • Superior vena cava syndrome
  • Chylothorax

Pleura

• Pleural effusion
• Pleural empyema
• Pleural fibrosis
• Pleural drain
• Mesothelioma

Diaphragm

• Diaphragm paralysis
• Congenital diaphragmatic hernias
• Acquired diaphragmatic hernias
• Diaphragmatic eventration