What is the anatomy of the pleural sac around the lungs? How is the mediastinum divided and what structures are found in each region? What are the functions of the pleural fluid and pleural recesses? Discover the answers to these questions and more in this detailed guide.

Understanding the Pleural Sac

The pleura is a serous membrane that forms a two-layered pleural sac around the lungs. The outer layer, called the parietal pleura, attaches to the chest wall, while the inner layer, the visceral pleura, covers the lungs, blood vessels, nerves, and bronchi. There is no anatomical connection between the right and left pleural cavities.

Regions of the Mediastinum

The mediastinum is the central compartment in the thoracic cavity between the pleural sacs of the lungs. It is divided into two major parts: the superior and inferior portions. The inferior portion is further divided into the anterior, middle, and posterior portions, each containing specific structures.

Superior Mediastinum

The superior mediastinum contains the thymus, trachea, esophagus, aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery, superior vena cava, brachiocephalic vein, thoracic duct, vagus nerve, left recurrent laryngeal nerve, cardiac nerve, and phrenic nerve.

Anterior Mediastinum

The anterior mediastinum contains the thymus and small arterial and venous branches, but no nerves.

Middle Mediastinum

The middle mediastinum contains the heart, pericardium, ascending aorta, pulmonary trunk, pericardiacophrenic arteries, superior vena cava, azygos vein, pulmonary vein, pericardiacophrenic vein, and the phrenic nerve.

Posterior Mediastinum

The posterior mediastinum contains the esophagus, thoracic aorta, azygos vein, hemiazygos vein, thoracic duct, and the vagus nerve.

Functions of the Pleural Fluid and Pleural Recesses

The pleural cavity contains a small amount of serous fluid that serves two key functions: it lubricates the pleural surfaces, allowing the lungs to slide easily during inflation and deflation, and it generates surface tension that pulls the visceral and parietal pleura together, enabling the thoracic cavity to expand during inspiration.

The pleural recesses, located posteriorly and anteriorly, are spaces where the pleural cavity is not completely filled by the lung parenchyma. These recesses, such as the costomediastinal and costodiaphragmatic recesses, provide a space for pleural effusions to accumulate.

Blood Supply and Innervation

The visceral pleura receives its blood supply from the bronchial circulation, while the parietal pleura is supplied by the intercostal arteries. The costal and cervical portions of the parietal pleura are innervated by the intercostal nerve, and the diaphragmatic portion is supplied by the phrenic nerve. The parietal pleura is the only part of the pleura that can sense pain, as the visceral pleura lacks sensory innervation.

Surgical Considerations

Pneumothorax, a condition where air enters the pleural space, is a common clinical event. Spontaneous pneumothorax, often seen in young male smokers, is caused by the presence of small blebs on the superior surface of the upper lobes. Traumatic pneumothorax can occur due to central line insertion, penetrating chest trauma, or rib fractures.

The treatment of pneumothorax depends on the size and presence of symptoms. Asymptomatic cases can often be observed, while larger or symptomatic pneumothoraces may require intervention such as chest tube placement or surgical repair.

Key Takeaways

1. The pleural sac is a two-layered membrane that surrounds the lungs, with the parietal pleura attached to the chest wall and the visceral pleura covering the lung structures.
2. The mediastinum is divided into superior, anterior, middle, and posterior regions, each containing specific anatomical structures.
3. The pleural fluid lubricates the pleural surfaces and generates surface tension to facilitate lung expansion during breathing.
4. Pleural recesses provide spaces for pleural effusions to accumulate.
5. The parietal pleura is innervated and can sense pain, while the visceral pleura lacks sensory innervation.
6. Pneumothorax, a condition where air enters the pleural space, can be spontaneous or traumatic and requires treatment based on the size and symptoms.