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.
- Definition: The mediastinum is the central space of the thoracic cavity located behind the sternum and between the two lungs and their respective pleura. 
|Compartments of the mediastinum |
|Structure||Boundaries||Organs||Vessels||Nerves||Causes of mediastinal masses|
|Inferior mediastinum||Anterior mediastinum|| |
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.
Vasculature of the mediastinum 
|Branches of aorta in thoracic cavity|
|Ascending aorta|| |
|Arch of the aorta|| || |
(Part of the descending aorta)
|Veins in the thoracic cavity|
|Superior vena cava (SVC)|| |
|Azygos venous system|| |
Accessory hemiazygos vein
| || |
See “” and “ .”
|Layers of pleura|
|Structure||Parietal pleura||Visceral pleura|
Pleural cavity 
- Location: potential space between the parietal and visceral pleura of the lungs
- Content: pleural fluid (secreted by serous membranes in the pleura)
- Costomediastinal recess: region where the mediastinal and costal pleura meet
- Costodiaphragmatic recess: region where the costal and diaphragmatic pleura meet
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).
- 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.
- Separates the thoracic cavity from the abdominal cavity
- Key muscle of inspiration
Parts of the diaphragm 
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
Central tendon of the diaphragm
- Location: point of convergence of the peripherally located muscles that make up the diaphragm
Structures passing through the diaphragm
|Openings of the diaphragm|
|Caval hiatus|| |
|Esophageal hiatus|| |
|Aortic hiatus|| |
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.
Vasculature and innervation of the diaphragm
|Venous drainage|| |
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.
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)
- Unilateral upper extremity swelling, pain, and weakness
- Unilateral facial and neck swelling, venous dilatation
- Retrosternal thyroid (substernal goiter)
- Mediastinal tumors
- Mediastinal vessels