Corpus: Pleura

The term pleura refers to the two-layered mesothelial membrane in the thoracic cavity and is one of the body's serous membranes.

The pleura originates from the embryonic coelom and consists of two layers with a space in between:

• Visceral pleura: inner layer of the pleura that covers both lungs, also known as the pulmonary pleura. It transitions into the outer layer at the region of the lung hilum.
• Parietal pleura: outer layer of the pleura, lining the inner surface of the chest wall and the upper side of the diaphragm.
• Pleural cavity (pleural space): small gap between the two layers of the pleura, filled with a few milliliters of serous fluid, called pleural fluid, which allows smooth movement of the lungs during breathing.

The parietal pleura is further divided based on the areas it covers:

• Diaphragmatic pleura: Covers the diaphragm.
• Mediastinal pleura: Covers the mediastinum. The part that lies against the heart's covering (the pericardium) can be called the pericardial pleura.
• Costal pleura: Covers the inner surface of the ribs.

At the sides of the diaphragm and around the mediastinum, the pleura forms so-called recesses, which act as reserve areas, allowing the lungs to expand during inhalation. There are four main pleural recesses:

• Costodiaphragmatic recess
• Costomediastinal recess
• Phrenicomediastinal recess
• Vertebromediastinal recess

There are areas without pleural covering, known as pleura-free triangles. The part of the pleura that extends above the first rib is called the pleural dome.

2.1. Innervation

Only the parietal pleura has sensory innervation. The nerves supplying the diaphragmatic and mediastinal pleura run through the phrenic nerve, while the costal pleura is innervated by the intercostal nerves.

2.2. Blood supply

• The visceral pleura receives blood from the bronchial arteries.
• The parietal pleura is supplied by the intercostal arteries and the pericardiacophrenic artery.

Venous drainage follows the same pathway as the arteries, through veins of the same name.

The pleura's structure, with its serous layer and subserous layer, is similar to other serous membranes. The serous layer consists of:

• Epithelial layer: A single layer of flat mesothelial cells, giving the pleura its smooth surface. These mesothelial cells secrete a viscous, slippery serous fluid.
• Lamina propria: This layer contains blood vessels and lymphatic vessels. It also has many elastic fibers that allow the pleura to stretch and move during breathing (inhalation and exhalation).

The subserous layer, located below the serous layer, acts as a sliding layer. It is made of loose connective tissue and is rich in elastic and collagen fibers, helping with flexibility and movement.

The pleura functions as a sliding interface between the chest wall and the lungs. Due to the negative pressure within the pleural cavity (-5 cm H2O) and the capillary adhesion between the two layers of the pleura, the lungs move in sync with the chest wall and diaphragm.

If the negative pressure in the pleural cavity is lost, for example from an injury (e.g., pneumothorax), the lungs can no longer expand and contract with the movements of the chest wall. The affected lung will collapse.

An abnormal accumulation of fluid (blood, pus, exudate, etc.) in the pleural recesses is called a pleural effusion.