Learning Objective:

Understand the structure and function of the pleurae, including visceral and parietal layers, pleural cavity, recesses, and neurovascular supply, and recognize their clinical relevance.

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Pleurae Overview

The pleurae are serous membranes lining the lungs and thoracic cavity. They allow smooth and effortless respiration. This lesson outlines their structure, function, and clinical correlations.

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Structure of the Pleurae

There are two pleurae: one for each lung. They consist of a layer of simple squamous cells (the mesothelium) supported by connective tissue.

Each pleura has two parts:

• Visceral pleura – covers the lungs
• Parietal pleura – lines the thoracic cavity

The two layers are continuous at the hilum, forming the pleural cavity.

Parietal Pleura

The parietal pleura covers the internal thoracic surface and is thicker than the visceral layer. Subdivisions include:

• Mediastinal pleura – lateral mediastinum
• Cervical pleura – pleural extension into the neck
• Costal pleura – inner ribs, costal cartilages, intercostal muscles
• Diaphragmatic pleura – thoracic surface of diaphragm

Visceral Pleura

The visceral pleura covers the lungs and extends into the interlobar fissures. It is continuous with the parietal pleura at the hilum.

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Pleural Cavity

The pleural cavity is a potential space between the visceral and parietal pleura containing serous fluid. This fluid:

• Lubricates pleural surfaces for smooth movement
• Generates surface tension to pull the pleurae together, allowing lung expansion

Pneumothorax occurs when air enters this cavity, disrupting surface tension.

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Pleural Recesses

Areas of the pleural cavity not fully occupied by lungs, forming:

• Costodiaphragmatic recess – between the costal and diaphragmatic pleurae
• Costomediastinal recess – between the costal and mediastinal pleurae

These are important clinically as sites where pleural fluid may accumulate.

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Neurovascular Supply

Parietal pleura:

Sensitive to pressure, pain, and temperature. Innervated by phrenic and intercostal nerves. Arterial supply from intercostal arteries.

Visceral pleura:

Insensitive to pain, temperature, or touch; senses only stretch. Receives autonomic innervation from the pulmonary plexus. Arterial supply from bronchial arteries.

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Clinical Relevance: Pneumothorax

A pneumothorax occurs when air enters the pleural space, removing surface tension and reducing lung expansion.

Clinical features:

chest pain, shortness of breath, asymmetrical chest expansion, and hyper-resonant percussion.

Types:

• Spontaneous – primary (no underlying lung disease), secondary (with lung disease)
• Traumatic – due to blunt or penetrating trauma, e.g., rib fracture

Treatment:

Small primary pneumothoraces may require minimal intervention; larger or secondary/traumatic cases need decompression via chest drain.

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