Parietal pleura

The parietal pleura is the outer layer of the pleura, a double-layered serous membrane that surrounds the lungs and lines the thoracic cavity. It is a thin, transparent membrane composed of mesothelial cells supported by connective tissue. The parietal pleura plays a crucial role in enclosing and protecting the lungs while facilitating smooth respiratory movements.

This layer is firmly attached to the inner surfaces of the thoracic wall, diaphragm, and mediastinum, forming a protective lining. It produces a small amount of pleural fluid that lubricates the pleural cavity, reducing friction during lung expansion and contraction. The parietal pleura is highly sensitive to pain, pressure, and temperature due to its somatic nerve supply, which makes it capable of detecting irritation or inflammation.

Location

The parietal pleura is located within the thoracic cavity, lining the inner surface of the rib cage, the diaphragm, and the mediastinum. It is divided into regions based on its anatomical location: costal pleura (lining the ribs), diaphragmatic pleura (covering the diaphragm), mediastinal pleura (adjacent to the mediastinum), and cervical pleura (extending above the first rib into the neck).

Anatomy

Layers of the Parietal Pleura

The parietal pleura is composed of two primary layers:

• Mesothelial Layer – The outermost layer, made up of simple squamous epithelium (mesothelium), which produces pleural fluid for lubrication.
• Submesothelial Connective Tissue – The underlying layer that provides structural support and houses blood vessels, lymphatics, and nerves.

Regions of the Parietal Pleura

The parietal pleura is divided into distinct regions based on its anatomical location:

1. Costal Pleura

• Lines the inner surface of the ribs and the intercostal muscles.
• Closely adheres to the thoracic wall.
• Receives innervation from the intercostal nerves, making it sensitive to pain and pressure.

2. Diaphragmatic Pleura

• Covers the superior surface of the diaphragm.
• Separated from the diaphragm by a layer of connective tissue called endothoracic fascia.
• Innervated by the phrenic nerve, providing sensory feedback related to the diaphragm.

3. Mediastinal Pleura

• Lines the lateral walls of the mediastinum, which contains the heart, great vessels, esophagus, and trachea.
• Forms the pulmonary ligament at the lung root, allowing free movement of structures during breathing.
• Supplied by the phrenic nerve for sensory innervation.

4. Cervical Pleura (Cupula)

• Extends above the first rib into the root of the neck.
• Reinforced by Sibson’s fascia (suprapleural membrane) to provide structural support.
• Particularly vulnerable to injury due to its position above the rib cage.

Pleural Reflections and Recesses

The parietal pleura forms folds or reflections where it changes direction, creating pleural recesses that allow lung expansion during deep breathing.

Pleural Reflections

• Mark the boundaries of the pleural cavity and connect different regions of the parietal pleura.
• Notable reflections include the sternal reflection (at the anterior thorax) and the vertebral reflection (at the posterior thorax).

Pleural Recesses

• Costodiaphragmatic Recess – Located at the junction of the costal and diaphragmatic pleurae, allowing lung expansion during inhalation.
• Costomediastinal Recess – Positioned near the sternum, providing additional space for lung expansion.

These recesses are potential spaces where fluid or air can accumulate, making them clinically significant for procedures like thoracocentesis.

Blood Supply

The parietal pleura receives its blood supply from:

• Intercostal Arteries – Supply the costal pleura.
• Internal Thoracic Arteries – Provide blood to the mediastinal pleura.
• Superior Phrenic Arteries – Supply the diaphragmatic pleura.
• Venous drainage is through intercostal veins, which empty into the azygos and internal thoracic veins.

Nerve Supply

The parietal pleura is richly innervated and highly sensitive to pain, temperature, and pressure due to somatic innervation.

• Intercostal Nerves – Supply the costal pleura and peripheral parts of the diaphragmatic pleura.
• Phrenic Nerve – Innervates the mediastinal pleura and central diaphragmatic pleura, making these regions especially pain-sensitive.

This nerve supply distinguishes the parietal pleura from the visceral pleura, which lacks somatic sensory innervation.

Lymphatic Drainage

Lymphatic drainage of the parietal pleura follows regional patterns:

• Costal and Cervical Pleura – Drain into intercostal lymph nodes and parasternal nodes.
• Diaphragmatic Pleura – Drains into the diaphragmatic nodes and posterior mediastinal nodes.
• Mediastinal Pleura – Drains into the mediastinal lymph nodes along the trachea and esophagus.

Microscopic Anatomy

Under the microscope, the parietal pleura consists of:

• Mesothelial Cells – Flattened cells that secrete pleural fluid, ensuring lubrication and reducing friction during breathing.
• Collagen and Elastic Fibers – Provide structural strength and elasticity.
• Capillaries and Lymphatics – Support nutrient exchange and drainage, preventing fluid buildup.

Pleural Fluid Production

The parietal pleura actively produces and regulates pleural fluid through its blood vessels and lymphatics. This fluid minimizes friction and maintains a pressure gradient essential for lung inflation.

Function of the Parietal Pleura

Protection of the Lungs

The parietal pleura acts as a protective layer that lines the thoracic cavity and shields the lungs from mechanical damage.

• It forms a physical barrier between the lungs and the chest wall, preventing direct contact with surrounding structures.
• It protects the lungs from injuries, including impacts and pressure changes during breathing.
• The elasticity and flexibility of the parietal pleura allow it to adapt to lung movements without compromising structural integrity.

Secretion of Pleural Fluid

The parietal pleura produces and regulates pleural fluid, which plays a critical role in lung function.

• Lubrication – The fluid reduces friction between the parietal and visceral pleurae, enabling smooth lung expansion and contraction during breathing.
• Surface Tension Maintenance – It helps maintain the adhesion between the two pleural layers, preventing lung collapse by creating a negative pressure within the pleural cavity.
• Fluid Balance Regulation – The parietal pleura absorbs excess fluid through its lymphatic vessels, preventing fluid accumulation and maintaining optimal pleural volume.

Facilitating Lung Expansion and Contraction

The parietal pleura supports respiratory mechanics by allowing the lungs to expand and contract without friction.

• The smooth surface and lubrication provided by the pleural fluid allow the lungs to glide against the chest wall during respiration.
• It maintains a negative pressure gradient in the pleural cavity, which aids lung inflation and prevents alveolar collapse during exhalation.
• It works with the diaphragm and rib movements to enable efficient ventilation.

Pressure Regulation

The parietal pleura helps maintain the intrapleural pressure required for proper lung expansion.

• During inspiration, the chest cavity expands, and the parietal pleura stretches outward, creating a negative pressure that draws air into the lungs.
• During expiration, the pressure increases slightly, aiding in passive recoil of the lungs to expel air.
• The negative pressure also prevents the lungs from collapsing by keeping them inflated against the thoracic wall.

Shock Absorption

The parietal pleura, along with the ribs and muscles, helps absorb mechanical shocks and distribute forces evenly across the thoracic cavity.

• Its flexibility allows it to withstand minor impacts without compromising the integrity of the lungs or thoracic structures.
• It acts as a cushion, minimizing damage during trauma or rapid pressure changes.

Sensory Perception and Reflexes

The parietal pleura is highly sensitive to pain, temperature, and pressure due to its somatic nerve supply.

• It detects inflammation, injury, or pressure changes, sending signals to the brain for protective responses.
• The sensitivity of the parietal pleura triggers reflex actions, such as coughing or changes in breathing patterns, to protect the lungs from harm.
• This sensory input is crucial for diagnosing conditions like pleuritis or pneumothorax, where pain indicates pleural irritation or damage.

Compartmentalization of the Thoracic Cavity

The parietal pleura divides the thoracic cavity into distinct compartments, ensuring structural organization and reducing the spread of infections or diseases.

• It separates the lungs from the heart, great vessels, and other mediastinal structures.
• It helps localize infections or fluids (e.g., pleural effusion) within specific regions, making it easier to diagnose and treat conditions.

Lymphatic Drainage

The parietal pleura supports lymphatic drainage, helping remove excess pleural fluid and foreign particles.

• It prevents the accumulation of fluid or debris in the pleural cavity, maintaining a healthy pleural environment.
• It absorbs proteins, cells, and pathogens through its lymphatic capillaries, aiding immune responses and tissue repair.
• This function is critical in preventing pleural effusion and edema.

Support for Respiratory Movements

The parietal pleura enhances respiratory efficiency by maintaining lung-chest wall adhesion and facilitating smooth lung movements.

• It prevents the lungs from collapsing during exhalation by creating surface tension with the visceral pleura.
• It enables lung expansion during inspiration by transmitting the movement of the chest wall and diaphragm to the lungs.
• Its elasticity accommodates variations in lung volume, allowing for deep breaths during exercise or stress.

Defense Against Infections and Inflammation

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

• It contains immune cells, including macrophages, that detect and eliminate pathogens.
• It produces inflammatory responses to contain infections and support healing.
• It plays a role in trapping and draining pathogens through the lymphatic system, limiting the spread of infections to other areas.

Clinical significance

The parietal pleura is clinically significant due to its role in respiratory mechanics and its sensitivity to pain and irritation. Pleuritis (pleurisy), an inflammation of the pleura, often caused by infections, autoimmune diseases, or trauma, leads to sharp chest pain during breathing due to friction between the inflamed pleural layers.

Pleural effusion, the accumulation of excess fluid in the pleural cavity, can result from infections, heart failure, or malignancies, leading to breathing difficulties and requiring drainage through thoracentesis. Pneumothorax, the presence of air in the pleural cavity, causes lung collapse and is often treated with chest tube insertion to remove trapped air.

Other conditions include mesothelioma, a rare cancer arising from pleural cells, and hemothorax, the collection of blood in the pleural space following trauma. Imaging techniques such as X-rays, CT scans, and ultrasound are commonly used for diagnosis, while treatments may include medications, drainage, or surgery, depending on the severity.