M01.05.021 Pleurae

The pleurae are vital serous membranes that play an essential role in respiration by lining the lungs and thoracic cavity. They allow for smooth and efficient lung movement during breathing. This article outlines the structure, function, and clinical correlations of the pleurae.

Structure of the Pleurae

The pleurae are serous membranes, composed of simple squamous epithelial cells (mesothelium) supported by connective tissue. There are two pleurae associated with each lung, each consisting of two parts:

1. Visceral Pleura

• Location: Covers the lungs’ outer surface, extending into the interlobar fissures.
• Function: Helps in maintaining lung shape and aids in lung expansion during respiration.

2. Parietal Pleura

• Location: Lines the internal surface of the thoracic cavity.
• Subdivisions:
  • Mediastinal Pleura: Covers the lateral aspect of the mediastinum.
  • Cervical Pleura: Extends into the neck.
  • Costal Pleura: Lines the inner aspect of the ribs and intercostal muscles.
  • Diaphragmatic Pleura: Covers the superior surface of the diaphragm.

Pleural Cavity

• Definition: The pleural cavity is a potential space between the visceral and parietal pleurae.
• Function of Serous Fluid: The cavity contains a small volume of serous fluid with two major functions:
  1. Lubrication: Reduces friction between pleural layers during respiration.
  2. Surface Tension: Helps maintain the lung’s expansion by pulling the visceral and parietal pleurae together.

Clinical Note: Pneumothorax

If air enters the pleural cavity, surface tension is lost, resulting in pneumothorax, a collapsed lung. This condition can impair lung expansion and cause difficulty breathing.

Pleural Recesses

The pleural cavity is not filled by the lungs, especially in the anterior and posteroinferior regions, forming two key recesses:

1. Costodiaphragmatic Recess: Between the costal and diaphragmatic pleurae.
2. Costomediastinal Recess: Between the costal and mediastinal pleurae, located behind the sternum.

Clinical Relevance of Pleural Recesses:

These recesses can collect fluid in conditions like pleural effusion.

Neurovascular Supply of the Pleurae

1. Parietal Pleura

• Sensory Supply: Sensitive to pain, pressure, and temperature (innervated by the phrenic and intercostal nerves).
• Blood Supply: Arterial blood is supplied by the intercostal arteries.

2. Visceral Pleura

• Sensory Supply: Not sensitive to pain or temperature; detects only stretch.
• Autonomic Supply: Innervated by the pulmonary plexus (sympathetic and vagus nerve fibers).
• Blood Supply: Supplied by the bronchial arteries (branches of the descending aorta).

Clinical Relevance: Pneumothorax

Types of Pneumothorax:

1. Spontaneous Pneumothorax
   • Primary: Occurs without underlying lung disease.
   • Secondary: Occurs in individuals with pre-existing lung conditions (e.g., COPD, asthma).
2. Traumatic Pneumothorax
   • Caused by chest trauma, such as rib fractures or stab wounds.

Symptoms:

• Chest pain
• Shortness of breath
• Asymmetrical chest expansion
• Hyper-resonance on percussion of the affected side

Treatment:

• Primary Pneumothorax: Often resolves on its own, and may require minimal intervention.
• Secondary and Traumatic Pneumothorax: May require chest drain insertion to remove excess air and allow lung re-inflation.

Key Points to Remember

• The pleurae are serous membranes that consist of two parts: visceral and parietal.
• The pleural cavity contains serous fluid that provides lubrication and maintains lung expansion.
• Pleural recesses provide potential spaces for fluid accumulation, which is important in conditions like pleural effusion.
• The neurovascular supply differs between the parietal and visceral pleurae, with the parietal pleura being sensitive to pain.
• Pneumothorax is a clinical condition where air enters the pleural cavity, causing lung collapse.