Pleural cavity

The pleural cavity is a potential space situated between the two layers of the pleura—the parietal pleura and the visceral pleura. It surrounds each lung and plays a vital role in respiration by providing a low-friction environment for lung expansion and contraction. The cavity contains a small amount of serous fluid, which lubricates the pleural surfaces, allowing smooth movement during breathing.

Location

There are two pleural cavities, one on each side of the mediastinum, within the thoracic cavity. Each pleural cavity surrounds one lung and lies between the thoracic wall and the lung surface. The cavities do not communicate with each other and are entirely independent, providing anatomical isolation.

Structure

The pleural cavity is enclosed by two continuous layers of serous membrane:

• Parietal pleura: Lines the internal surface of the thoracic wall, diaphragm, mediastinum, and superior aspect of the lung apex (within the root of the neck)
• Visceral pleura: Covers the external surface of the lung and dips into its fissures

The space between these two layers is the pleural cavity. Under normal conditions, it contains a thin film of pleural fluid (~10–20 mL), secreted by mesothelial cells, that prevents friction during respiratory movements.

Subdivisions of the Parietal Pleura

The parietal pleura is further subdivided based on the structures it lines:

• Costal pleura: Covers the inner surface of the ribs, costal cartilages, and intercostal muscles
• Diaphragmatic pleura: Lines the superior surface of the diaphragm
• Mediastinal pleura: Covers the lateral aspect of the mediastinum
• Cervical pleura (cupula): Extends into the root of the neck, covering the apex of the lung above the first rib

These subdivisions are continuous with each other, forming a continuous parietal membrane that transitions into visceral pleura at the lung root (hilum).

Recesses of the Pleural Cavity

Certain areas of the pleural cavity form recesses where the lungs do not completely fill the pleural space during quiet respiration:

• Costodiaphragmatic recess: Located between the costal and diaphragmatic pleurae; the most dependent part of the pleural cavity in an upright individual
• Costomediastinal recess: Lies anteriorly between the costal and mediastinal pleura; more pronounced on the left due to the cardiac notch

These recesses allow for lung expansion during deep inspiration and serve as fluid collection points in pathological conditions.

Function

• Facilitates frictionless movement of the lungs during breathing
• Maintains negative pressure that keeps the lungs inflated
• Acts as a physical barrier separating lungs from thoracic structures
• Provides protective cushioning for the lungs

The negative intrapleural pressure generated within the pleural cavity during respiration is essential for drawing air into the lungs and keeping them adhered to the chest wall.

Pleural Fluid

• Produced by: Mesothelial cells lining the pleura
• Functions: Lubrication, surface tension maintenance, and pleural cavity hydration
• Volume: Normally ~10–20 mL per cavity; continuously reabsorbed and replenished

Surface Projections and Borders

The pleura extends beyond the margins of the lungs, particularly at the inferior border:

• Midclavicular line: Lung reaches rib 6; pleura extends to rib 8
• Midaxillary line: Lung reaches rib 8; pleura extends to rib 10
• Paravertebral line: Lung reaches T10; pleura extends to T12

These anatomical landmarks are clinically relevant for procedures like thoracentesis.

Innervation

• Parietal pleura: Somatic innervation
  • Costal pleura – intercostal nerves
  • Diaphragmatic and mediastinal pleura – phrenic nerve
• Visceral pleura: Autonomic innervation from pulmonary plexuses; no somatic pain sensation

Due to somatic innervation, the parietal pleura is highly sensitive to pain, pressure, and temperature. In contrast, the visceral pleura is insensitive to pain.

Blood Supply

• Parietal pleura: Intercostal arteries (costal), pericardiacophrenic arteries (mediastinal), and musculophrenic arteries (diaphragmatic)
• Visceral pleura: Bronchial arteries (same as lung tissue)

Lymphatic Drainage

• Parietal pleura: Drains into intercostal, parasternal, and mediastinal lymph nodes
• Visceral pleura: Drains into pulmonary and bronchopulmonary nodes

Clinical Relevance

• Pleural effusion: Accumulation of excess fluid in the pleural cavity, which may be serous (transudate), inflammatory (exudate), or blood (hemothorax)
• Pneumothorax: Air in the pleural cavity causing lung collapse
• Pleuritis: Inflammation of the pleura, often causing sharp chest pain during respiration
• Thoracentesis: Procedure to aspirate fluid from the pleural cavity; typically performed in the costodiaphragmatic recess above the rib to avoid neurovascular bundle