Pleura

The pleura is a double-layered serous membrane that surrounds the lungs and lines the thoracic cavity. It provides a protective covering for the lungs and facilitates smooth respiratory movements. The pleura is divided into two layers—parietal pleura and visceral pleura—which are separated by the pleural cavity filled with a thin layer of pleural fluid. This fluid reduces friction during breathing and helps maintain lung expansion.

The parietal pleura lines the inner surface of the thoracic wall, diaphragm, and mediastinum, while the visceral pleura closely adheres to the surface of the lungs, including fissures between lobes. Both layers are continuous at the hilum, where the bronchi, blood vessels, and nerves enter the lungs. The pleura also acts as a barrier to infections and supports the mechanical function of breathing by maintaining a pressure gradient within the pleural cavity.

Location

The pleura is located within the thoracic cavity, surrounding both lungs. The parietal pleura lines the thoracic wall, diaphragm, mediastinum, and extends into the neck, while the visceral pleura covers the lungs’ surface, including their fissures. The pleural cavity between the layers is filled with pleural fluid and lies adjacent to the ribs, spine, and heart within the chest.

Anatomy

Layers of the Pleura

1. Parietal Pleura

The parietal pleura is the outer layer, attached to the walls of the thoracic cavity. It is subdivided into regions based on its anatomical location:

• Costal Pleura – Lines the inner surface of the rib cage and intercostal muscles.
• Diaphragmatic Pleura – Covers the superior surface of the diaphragm.
• Mediastinal Pleura – Lines the mediastinum, including the structures between the lungs such as the heart and major blood vessels.^[6]
• Cervical Pleura (Cupula) – Extends above the first rib into the root of the neck.

The parietal pleura is highly sensitive to pain due to its somatic nerve supply.

2. Visceral Pleura

The visceral pleura is the inner layer that tightly adheres to the surface of the lungs, including the fissures between lobes. Unlike the parietal pleura, it has autonomic innervation and is insensitive to pain, although it responds to stretch.

This layer provides structural protection and moves with the lungs during respiration.

Pleural Cavity

The pleural cavity is the potential space between the parietal and visceral pleura.

• Filled with pleural fluid secreted by mesothelial cells.
• Maintains negative pressure, which is crucial for lung expansion during inhalation.
• Prevents lung collapse by maintaining adhesion between the pleural layers.

Pleural Recesses

The pleura forms recesses where layers reflect and create spaces for lung expansion during deep inspiration:

• Costodiaphragmatic Recess – Located at the junction of the costal and diaphragmatic pleura. It is the largest recess and can accommodate lung expansion during deep breaths.
• Costomediastinal Recess – Located near the sternum, between the costal and mediastinal pleura.

These recesses are potential spaces for fluid accumulation and are important for diagnostic procedures like thoracentesis.^[4]

Microscopic Anatomy

The pleura is composed of the following layers:

• Mesothelium – The outermost layer consisting of simple squamous epithelial cells. These cells produce pleural fluid for lubrication.
• Submesothelial Connective Tissue – Supports the mesothelium and contains collagen fibers, elastic fibers, blood vessels, and lymphatics.

The pleura’s thin structure and permeability allow for efficient fluid and nutrient exchange while preventing excess fluid buildup.

Blood Supply

• Parietal Pleura – Supplied by:
  • Intercostal arteries for the costal pleura.
  • Internal thoracic arteries for the mediastinal pleura.
  • Superior phrenic arteries for the diaphragmatic pleura.
• Visceral Pleura – Supplied by:
  • Bronchial arteries, which also supply lung tissue.

Venous drainage of both layers occurs through the intercostal veins and pulmonary veins.

Nerve Supply

• Parietal Pleura – Innervated by somatic nerves, making it sensitive to pain, pressure, and temperature:
  • Intercostal nerves supply the costal pleura.
  • Phrenic nerve supplies the mediastinal and diaphragmatic pleura.
• Visceral Pleura – Receives autonomic innervation from the vagus nerve and sympathetic fibers. It lacks pain receptors but can detect stretch or distention.

Lymphatic Drainage

The pleura has an extensive lymphatic network to prevent fluid buildup and assist in immune defense.

• The parietal pleura drains into intercostal nodes, parasternal nodes, and diaphragmatic nodes.
• The visceral pleura drains into the pulmonary lymph nodes and bronchopulmonary nodes near the lung hilum.

Lymphatic drainage plays a crucial role in removing excess fluid and preventing pleural effusion.

Pleural Reflections

Pleural reflections are folds where the pleura changes direction, marking anatomical boundaries.

• Sternal Reflection – Occurs along the anterior thorax where the costal pleura transitions to the mediastinal pleura.^[1]
• Vertebral Reflection – Posterior fold where the costal pleura meets the mediastinal pleura.
• These reflections create potential spaces for fluid accumulation, often evaluated in imaging studies.

Pleural Fluid

Pleural fluid, secreted by mesothelial cells, has the following characteristics:

• Volume – Typically 10–20 mL within the pleural cavity.
• Composition – Contains proteins, glucose, and immune cells.
• Function – Reduces friction, facilitates lung movement, and maintains a pressure gradient for lung expansion.

This fluid is continuously absorbed and replenished through capillaries and lymphatics, maintaining homeostasis.

Function

Protection of the Lungs

The pleura acts as a protective covering for the lungs, shielding them from mechanical injuries and friction caused by movement within the thoracic cavity.

• The parietal pleura lines the thoracic cavity and forms a barrier against external impacts.
• The visceral pleura adheres tightly to the lung surface, providing an additional layer of protection to the lung tissue.
• Both layers prevent the lungs from direct contact with surrounding structures, minimizing damage from mechanical forces.

Secretion and Regulation of Pleural Fluid

The pleura produces and maintains a thin layer of pleural fluid within the pleural cavity, which serves multiple purposes:

• Lubrication – Reduces friction between the parietal and visceral pleura, allowing smooth movement during respiration.^[8]
• Cohesion – Creates surface tension that keeps the lungs attached to the chest wall, enabling expansion during inhalation and preventing lung collapse.
• Fluid Balance – Regulates fluid levels through capillary filtration and lymphatic drainage, preventing fluid accumulation and maintaining optimal pleural pressure.

Facilitating Lung Expansion and Contraction

The pleura plays a crucial role in respiration by aiding lung movements during inhalation and exhalation.

• The parietal pleura is firmly attached to the chest wall and diaphragm. As the thoracic cavity expands, it pulls on the pleural fluid, which in turn moves the visceral pleura along with the lungs.
• This synchronized movement ensures the lungs expand fully during inspiration and recoil effectively during expiration, maintaining proper ventilation.

Maintaining Negative Pressure

The pleura helps sustain negative pressure within the pleural cavity, which is essential for lung inflation.

• During inspiration, the chest wall expands, pulling the parietal pleura outward and increasing the negative pressure in the pleural cavity. This negative pressure allows the lungs to inflate as air flows in.^[7]
• During expiration, the pressure returns to baseline, enabling lung recoil and air expulsion.
• This mechanism prevents lung collapse and maintains continuous airflow.

Shock Absorption

The pleura, along with the pleural fluid, acts as a shock absorber to protect the lungs from sudden impacts or mechanical stress.

• The fluid’s cushioning effect minimizes vibrations and distributes pressure evenly across the pleural cavity.
• This feature is especially important during activities like coughing, sneezing, and sudden chest movements.

Compartmentalization of the Thoracic Cavity

The pleura divides the thoracic cavity into separate compartments, ensuring structural organization and isolating the lungs.

• It prevents the spread of infections or diseases between the lungs by maintaining individual pleural cavities.
• If one lung is affected by conditions such as pneumothorax or pleural effusion, the other lung can continue functioning, minimizing respiratory compromise.

Pressure Regulation and Fluid Dynamics

The pleura assists in fluid regulation and pressure balance to support lung function.

• Excess pleural fluid is drained through lymphatic vessels, maintaining a stable volume within the cavity.
• This drainage prevents pleural effusion and maintains optimal respiratory mechanics.

Sensory Feedback and Pain Sensation

The parietal pleura contains somatic nerve fibers that provide sensory feedback, making it highly sensitive to pain, pressure, and temperature changes.^[5]

• This feedback triggers reflexes, such as coughing or shallow breathing, to protect the lungs during injury or inflammation.
• The visceral pleura, in contrast, has autonomic nerve fibers and is insensitive to pain, allowing it to adapt to lung movements without discomfort.

Supporting Immune Defense

The pleura contributes to immune defense by acting as a barrier against infections and foreign particles.

• Macrophages within the pleural cavity detect and destroy pathogens.^[3]
• Lymphatic drainage clears cellular debris and microorganisms, preventing infection buildup.
• The pleural fluid supports immune cells, enabling localized responses to inflammation or injury.

Facilitating Diagnostic and Therapeutic Procedures

The pleura provides accessible spaces for medical procedures:

• Thoracentesis – The pleural cavity allows fluid removal for diagnostic testing or relief from pleural effusion.
• Pleural Biopsy – Enables sampling of pleural tissue for detecting infections, cancers, or inflammatory diseases.
• Chest Tube Insertion – Used to drain air, blood, or fluid from the pleural cavity in emergencies like pneumothorax or hemothorax.

Aiding in Lung Compliance

The elasticity of the pleura contributes to lung compliance, or the ability of the lungs to expand and contract effectively.

• Elastic fibers within the pleura allow it to stretch and adapt to changes in lung volume during deep breathing or physical activity.
• This flexibility reduces the work required for breathing, improving overall respiratory efficiency.

Pressure Equalization

The pleura helps maintain pressure equilibrium during respiration:

• The pleural fluid minimizes resistance between layers, ensuring smooth sliding movements as the lungs expand and contract.^[2]
• The pleural recesses, such as the costodiaphragmatic recess, allow the lungs to expand further during deep inspiration, supporting increased oxygen intake.

Clinical Significance

The pleura is clinically significant because it can be affected by various diseases and conditions that impact breathing and lung function. Pleuritis (pleurisy) is an inflammation of the pleura, often caused by infections, autoimmune disorders, or trauma, leading to sharp chest pain during breathing due to friction between the pleural layers.

Pleural effusion, the abnormal accumulation of fluid in the pleural cavity, may result from heart failure, infections, or malignancies, causing shortness of breath and requiring drainage through thoracentesis. Pneumothorax occurs when air enters the pleural cavity, leading to lung collapse, while hemothorax involves blood accumulation, often caused by trauma or surgery.

Mesothelioma, a rare but aggressive cancer, arises from the pleural lining and is often linked to asbestos exposure. Diagnosis of pleural conditions typically involves imaging techniques like X-rays, CT scans, and pleural biopsy, with treatments ranging from medications and drainage procedures to surgery in severe cases.