The Pleura: Structure, Function, and Clinical Significance

Pleural Anatomy

The pleura is a serous membrane with two continuous layers:

• Visceral pleura: Adheres to lung surface; extends into fissures; insensitive to pain (visceral innervation).
• Parietal pleura: Lines thoracic wall, diaphragm, mediastinum; highly sensitive (somatic innervation – intercostal and phrenic nerves).

The pleural space (cavity) is a potential space between layers, containing ~0.1–0.2 mL/kg of serous pleural fluid (transudate) that lubricates and couples the layers via surface tension.

Key Functions

1. Mechanical coupling: Negative intrapleural pressure (~–5 cm H₂O at rest) keeps lungs expanded against chest wall.
2. Frictionless movement: Fluid allows smooth sliding during respiration.
3. Compartmentalization: Separates lungs from mediastinum and contralateral hemithorax.
4. Immune defense: Mesothelial cells, macrophages, lymphocytes in fluid respond to infection/injury.

Clinical Correlates

Condition	Pathophysiology	Key Features
Pleuritis (pleurisy)	Pleural inflammation	Sharp pleuritic pain, friction rub
Pleural effusion	Fluid accumulation (>physiologic)	Transudate (CHF, cirrhosis) vs exudate (infection, malignancy) by Light's criteria
Pneumothorax	Air in pleural space	Loss of negative pressure → lung collapse; tension = emergency
Hemothorax	Blood in pleural space	Trauma, malignancy, post-op
Empyema	Pus in pleural space	Complicated parapneumonic effusion; requires drainage
Mesothelioma	Malignant pleural tumor	Asbestos exposure; poor prognosis

Diagnostic Tools

• Chest X-ray (upright, lateral decubitus)
• Ultrasound (bedside, sensitive for fluid)
• CT chest (characterization, loculations)
• Thoracentesis (diagnostic/therapeutic) with fluid analysis