Pleura

1. Overview

The pleura is a double-layered serous membrane that envelops the lungs and lines the thoracic cavity. It plays a critical role in lung expansion, protection, and maintenance of negative intrathoracic pressure. The pleura is divided into two parts: the visceral pleura (covering the lungs) and the parietal pleura (lining the thoracic wall, diaphragm, and mediastinum). The space between them, the pleural cavity, contains a thin layer of fluid essential for respiration.

2. Location

The pleura is located in the thoracic cavity, encompassing:

• Visceral pleura: Tightly adherent to the surface of the lungs, extending into fissures between lobes

• Parietal pleura: Lines the inner surface of the thoracic wall, diaphragm, and lateral aspect of the mediastinum

The space between these two layers—the pleural cavity—extends from the thoracic inlet at the top to the costodiaphragmatic recesses at the base.

3. Structure

Each pleural layer is composed of:

• Mesothelium: A single layer of flattened, squamous epithelial cells

• Underlying connective tissue: Contains blood vessels, lymphatics, and nerves

Visceral pleura: Lacks pain fibers and receives autonomic innervation from the pulmonary plexus.
Parietal pleura: Innervated by somatic nerves (intercostal and phrenic nerves), and thus is sensitive to pain, pressure, and temperature.

Subdivisions of Parietal Pleura:

• Costal pleura: Lines the inner aspect of the ribs and intercostal spaces

• Diaphragmatic pleura: Covers the thoracic surface of the diaphragm

• Mediastinal pleura: Lines the lateral aspect of the mediastinum

• Cervical pleura (cupula): Extends into the neck above the first rib

4. Function

The pleura performs essential mechanical and physiological functions for respiration:

• Lubrication: Pleural fluid reduces friction between lung surfaces and the thoracic wall during breathing

• Creates negative pressure: Allows the lungs to stay inflated by maintaining a vacuum in the pleural cavity

• Compartmentalization: Helps localize infections or fluid collections within each hemithorax

• Facilitates lung movement: Ensures smooth expansion and contraction with thoracic motion

5. Physiological Role(s)

The pleura contributes to normal respiratory physiology in multiple ways:

• Ventilation mechanics: Transmits thoracic cage expansion to lung tissue via cohesive forces of the pleural fluid

• Mucosal immunity: The mesothelium provides local immune surveillance and secretes cytokines

• Lymphatic drainage: Particularly in the parietal pleura, removes excess fluid and particulates from the pleural space

• Surface tension regulation: Pleural fluid creates capillary forces that assist lung recoil

6. Clinical Significance

Pleural Effusion

Accumulation of excess fluid in the pleural cavity due to infection, heart failure, malignancy, or inflammation. Classified as:

• Transudative: Clear, low-protein fluid (e.g., congestive heart failure)

• Exudative: Protein-rich fluid (e.g., pneumonia, TB, cancer)

Symptoms: Dyspnea, chest heaviness, reduced breath sounds. Diagnosed via chest X-ray, ultrasound, or CT; treated with thoracentesis or chest tube drainage.

Pleuritis (Pleurisy)

Inflammation of the pleura, often viral or autoimmune. Sharp, stabbing chest pain worsens with breathing or coughing. Best appreciated on auscultation as a pleural rub. Managed with anti-inflammatories or antibiotics depending on cause.

Pneumothorax

Air in the pleural space causes lung collapse. Can be spontaneous (young, thin individuals), traumatic, or iatrogenic (e.g., from central line insertion). Presents with sudden chest pain and dyspnea. Requires emergency decompression or chest tube insertion.

Hemothorax

Blood accumulation in the pleural cavity, usually due to trauma or ruptured vessels. May require chest drainage and surgical intervention if ongoing bleeding occurs.

Chylothorax

Presence of lymphatic fluid (chyle) in the pleural space due to injury or obstruction of the thoracic duct. Presents as a milky pleural effusion; managed by drainage and dietary modification or surgery.

Malignant Pleural Disease

Cancers such as mesothelioma or metastases (e.g., from lung or breast cancer) can involve the pleura. Symptoms include persistent pleural effusions, chest pain, and weight loss. Diagnosis via thoracentesis and pleural biopsy; treatment includes pleurodesis or chemotherapy.