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.
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).
- 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
The mediastinum is divided into four different compartments.
|Structure||Boundaries||Content||Causes of mediastinal masses|
|Superior mediastinum|| || |
|Inferior mediastinum||Anterior 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.
Vasculature of the mediastinum
|Ascending aorta|| |
|Arch of the aorta|| || |
(Part of the descending aorta)
- Formed by the junction between the left and right brachiocephalic veins
- Drains blood from structures above the diaphragm → right atrium of the heart
- The SVC can be divided into two main parts:
- Formed by the convergence of the jugular vein and the subclavian vein at the
- Receive blood from the chest wall, neck, esophagus, lungs, and pericardium
- The right brachiocephalic vein is short and is more vertically oriented, while the left brachiocephalic vein is long and has more horizontal orientation.
- The right and left vein merge to form the superior vena cava.
- 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
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.
|Pleural cavity|| |
|Parietal pleura|| |
|Visceral pleura|| |
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.
- 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|| |
|Central tendon of the diaphragm|| |
|Openings of the diaphragm|
Vasculature of the diaphragm
|Venous drainage|| |
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.
|Caval hiatus|| |
|Esophageal hiatus|| |
|Aortic hiatus|| |