Pleural Layers
The pleura has two layers — visceral and parietal — enclosing the potential pleural space. Their distinct innervation and blood supply explain the clinical features of pleural disease.
✦ The Mnemonic
"Very Careful Carvers Make Distinct Planes"
Visceral pleura · four parts of Parietal pleura (Cervical, Costal, Mediastinal, Diaphragmatic)
Clinical Breakdown
The pleural space normally contains 5–20 mL of serous fluid for lubrication. It is maintained at subatmospheric pressure — pneumothorax (air in pleural space) collapses the lung because this negative pressure is lost. A tension pneumothorax occurs when air enters the pleural space but cannot escape — progressive mediastinal shift compresses the contralateral lung and great vessels.
Pleural effusion: transudates (protein <25 g/L) from heart failure, cirrhosis, nephrotic syndrome; exudates (protein >35 g/L or Light's criteria) from infection, malignancy, pulmonary embolism. Light's criteria: exudate if pleural fluid protein/serum protein >0.5, LDH/serum LDH >0.6, or pleural LDH >2/3 upper limit of normal.
The cervical pleura (cupola) rises 2.5 cm above the medial third of the clavicle. This makes it vulnerable during: subclavian/internal jugular central venous cannulation, brachial plexus blocks, and neck dissection. Pneumothorax after a central line should always be checked for on CXR.
⭐ Clinical Pearl
Mesothelioma: malignant tumour of the parietal pleura, almost exclusively caused by asbestos exposure (latency 20–40 years). Presents with progressive breathlessness and chest wall pain from parietal pleural infiltration (somatic innervation explains the pain). Median survival 12–18 months from diagnosis.