Visceral Pleura

1. Overview

The visceral pleura is the inner layer of the pleural membrane that tightly adheres to the surface of the lungs. It forms part of the pleural sac, working alongside the parietal pleura (which lines the thoracic cavity) to create the pleural cavity. The visceral pleura plays a crucial role in facilitating lung movement, reducing friction, and maintaining negative intrathoracic pressure necessary for breathing.

2. Location

The visceral pleura:

• Directly covers the lungs, following the contours of all lung lobes and fissures

• Extends into the interlobar fissures, separating different lobes

• Is continuous with the parietal pleura at the hilum of the lung, where structures of the root (bronchi, vessels, nerves) enter and exit

This close anatomical relationship allows the lungs to expand and contract smoothly within the thoracic cavity during respiration.

3. Structure

The visceral pleura is a thin serous membrane composed of:

• Mesothelium: A single layer of flat, squamous epithelial cells

• Submesothelial connective tissue: Contains blood vessels, lymphatics, and a small number of elastic fibers

Unlike the parietal pleura, the visceral pleura:

• Is avascular to somatic innervation – it lacks pain sensitivity

• Receives autonomic nerve supply from the pulmonary plexus

Its smooth, moist surface is kept lubricated by pleural fluid, secreted in small amounts by mesothelial cells.

4. Function

The visceral pleura serves multiple important functions:

• Allows lung movement: Facilitates smooth sliding of the lung surface against the parietal pleura during breathing

• Maintains negative pressure: Works with the parietal pleura to keep the lungs inflated by maintaining intrapleural pressure lower than atmospheric pressure

• Barrier protection: Helps protect the lungs from mechanical trauma and infection

5. Physiological Role(s)

In terms of respiratory physiology, the visceral pleura plays roles in:

• Respiratory mechanics: Transmits thoracic wall expansion to lung tissue due to surface tension in pleural fluid

• Lymphatic drainage: Contains lymphatic vessels that aid in clearing fluid and particulate matter from the pleural space

• Immunologic surveillance: The mesothelial layer may produce cytokines and participate in local immune responses

• Regulation of pleural fluid dynamics: Balances fluid secretion and absorption to maintain proper lubrication without accumulation

6. Clinical Significance

Pleuritis (Pleurisy)

While the visceral pleura itself is insensitive to pain, inflammation often affects both visceral and parietal layers. Symptoms include:

• Pleuritic chest pain (due to parietal pleura involvement)

• Friction rub on auscultation

• Shallow breathing due to pain

Common causes include viral infections, autoimmune conditions, and pulmonary embolism.

Pleural Effusion

Excess fluid can accumulate between the visceral and parietal pleura due to:

• Congestive heart failure (transudate)

• Infection or malignancy (exudate)

The visceral pleura often appears thickened or inflamed in exudative effusions. Managed with thoracentesis and treatment of the underlying cause.

Pneumothorax

When air enters the pleural cavity, the negative pressure is lost, causing lung collapse. Types include:

• Spontaneous (rupture of visceral pleura)

• Traumatic

• Tension (life-threatening)

Management includes chest tube placement and sometimes pleurodesis.

Malignant Mesothelioma

A rare cancer originating from mesothelial cells, often linked to asbestos exposure. Involves the visceral pleura and leads to:

• Pleural thickening

• Effusions

• Dyspnea and chest pain

Prognosis is generally poor; management includes surgery, chemotherapy, and palliative care.

Visceral Pleural Invasion (VPI)

In lung cancer, tumor invasion of the visceral pleura (especially beyond the elastic layer) is a criterion for upstaging the disease (e.g., T2). VPI is associated with a worse prognosis and often affects treatment planning.