Pleural Cavity

1. Overview

The pleural cavity is a potential space located between the two layers of the pleura—the visceral pleura (covering the lungs) and the parietal pleura (lining the thoracic wall). It contains a small amount of lubricating fluid that facilitates frictionless lung movement during respiration. Although normally a virtual space, it can expand pathologically due to fluid, air, or blood accumulation, significantly impacting respiratory mechanics.

2. Location

The pleural cavity is located in the thoracic cavity, with one cavity on each side of the mediastinum. Each cavity surrounds a lung and is enclosed by the following boundaries:

• Medially: Mediastinum (containing heart, trachea, esophagus)

• Lateral boundary: Thoracic wall (parietal pleura)

• Superiorly: Cervical pleura (extends into root of the neck)

• Inferiorly: Diaphragmatic pleura

The right and left pleural cavities do not communicate with each other, which is clinically significant in cases of unilateral thoracic pathology.

3. Structure

The pleural cavity is not a true anatomical space under normal conditions but a narrow fluid-filled gap. Its structural components include:

• Visceral pleura: Covers the lungs and is firmly adherent to lung tissue

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

• Pleural fluid: A thin film (5–15 mL) of serous fluid secreted by mesothelial cells that reduces surface tension and allows lung movement during respiration

Specialized recesses within the cavity allow for lung expansion:

• Costodiaphragmatic recess: Located between the diaphragm and lower ribs; site of fluid accumulation

• Costomediastinal recess: Located anteriorly near the heart; more prominent on the left

4. Function

The pleural cavity serves multiple critical functions:

• Allows lung expansion and recoil: The negative pressure within the cavity keeps lungs inflated during the respiratory cycle

• Reduces friction: Pleural fluid acts as a lubricant, minimizing resistance between the moving pleural surfaces

• Compartmentalization: Helps isolate infections or injuries to one hemithorax

5. Physiological Role(s)

The pleural cavity plays essential roles in respiratory physiology:

• Maintains negative intrapleural pressure: This suction effect keeps the lungs expanded and allows passive lung inflation with chest wall expansion

• Fluid homeostasis: Pleural fluid is continually secreted and reabsorbed by mesothelial cells and lymphatics to maintain optimal surface tension

• Facilitates pulmonary mechanics: The close apposition of the pleural layers helps translate thoracic wall movement into lung expansion

6. Clinical Significance

Pleural Effusion

Abnormal accumulation of fluid in the pleural cavity. Types include:

• Transudative: Caused by systemic factors like heart failure or liver disease

• Exudative: Caused by local inflammation, malignancy, or infection

Symptoms: Dyspnea, chest heaviness, and decreased breath sounds. Managed with thoracentesis and treatment of the underlying cause.

Pneumothorax

Presence of air in the pleural cavity, disrupting negative pressure and causing lung collapse. Types:

• Spontaneous: Often in tall, young males without trauma

• Traumatic: From chest injury or procedures like central line placement

• Tension pneumothorax: Life-threatening variant with mediastinal shift; requires immediate needle decompression

Hemothorax

Blood in the pleural cavity, usually due to trauma or ruptured vessels. Requires chest tube placement to drain blood and prevent lung compression.

Empyema

Pus accumulation in the pleural cavity, often a complication of pneumonia. Managed with drainage (chest tube or surgery) and antibiotics.

Chylothorax

Accumulation of chyle (lymphatic fluid) in the pleural space due to injury or obstruction of the thoracic duct. Presents as a milky effusion.

Malignant Effusion

Metastatic cancers may invade the pleural cavity, leading to recurrent effusions. May require repeated drainage, pleurodesis, or pleurectomy.

Diagnostic and Therapeutic Thoracentesis

A procedure to aspirate pleural fluid from the costodiaphragmatic recess, used for diagnosis (e.g., cytology, biochemistry) or symptomatic relief.