Layers of the Pleura — Visceral and Parietal
Definition
The pleura is a serous membrane lined by flattened epithelium (mesothelium). The mesothelium secretes a watery lubricant — serous fluid — which fills the potential space between the two layers.
There are two pleural sacs — one on either side of the mediastinum. Each sac has two continuous layers:
- Visceral (pulmonary) pleura — inner layer, invests the lung
- Parietal pleura — outer layer, lines the thoracic cavity walls
The two layers are continuous with each other at the root (hilum) of the lung. Between them lies the pleural cavity — a slit-like potential space containing 5–10 ml of clear serous fluid.
Visceral (Pulmonary) Pleura
- Covers all surfaces of the lung including the fissures
- Absent only at the hilum and along the attachment of the pulmonary ligament
- Firmly adherent to the lung surface — cannot be separated from it in the living
- Thinner than the parietal pleura
- Behaves physiologically like the lung (expands and recoils with it)
Parietal Pleura
- Thicker than the visceral pleura
- Lines the walls of the pulmonary (thoracic) cavity
- Loosely attached to the thoracic wall by endothoracic fascia — a thin areolar layer easily separated from the wall in the living
- Divided into 4 parts based on the structure it lines (see Subdivisions of the Parietal Pleura)
Development Basis
| Layer | Develops from | Nerve Supply | Pain Sensitivity |
|---|---|---|---|
| Visceral pleura | Splanchnopleuric mesoderm | Autonomic (sympathetic T2–T5; parasympathetic — vagus) | Insensitive to pain |
| Parietal pleura | Somatopleuric mesoderm | Somatic nerves (intercostal + phrenic) | Sensitive to pain |
Comparison Table: Parietal vs Visceral Pleura
| Feature | Parietal Pleura | Visceral Pleura |
|---|---|---|
| Position | Lines thoracic wall, diaphragm, mediastinum | Lines surface of lung and fissures |
| Development | Somatopleuric mesoderm | Splanchnopleuric mesoderm |
| Nerve supply | Somatic (intercostal + phrenic nerves) | Autonomic (sympathetic + vagus) |
| Pain sensitivity | Sensitive — pleuritic pain + referred pain | Insensitive to pain |
| Blood supply | Intercostal + pericardiacophrenic vessels | Bronchial arteries |
| Lymphatic drainage | Intercostal lymph nodes | Tracheobronchial / bronchopulmonary nodes |
| Thickness | Thicker | Thinner |
Pleural Cavity
The pleural cavity is a potential space — in health it contains only 5–10 ml of clear serous fluid for lubrication. The fluid allows frictionless movement of the two layers during respiration.
Negative pressure: The pleural cavity normally has a slight sub-atmospheric (negative) pressure, which keeps the lung expanded against the thoracic wall. During inspiration, pressure becomes more negative and air is drawn in.
Pathological contents:
| Condition | Contents |
|---|---|
| Pleural effusion (hydrothorax) | Excess serous fluid |
| Pneumothorax | Air |
| Haemothorax | Blood |
| Pyothorax (empyema) | Pus |
| Chylothorax | Chyle |