Normal Anatomy and Pleural Fluid Physiology

Normal anatomy

The pleural space is walled by the parietal pleura which lines the inside of the chest wall, and the visceral pleura which lines the lung. Pleural fluid is produced by filtering from systemic capillaries within the parietal interstitium and production is greatest at the apex of the lung [5].

The fluid enters the pleural space down a small pressure gradient. In health, approximately 0.13 mls/kg bodyweight of pleural fluid lubricates the space between the parietal and visceral pleura [6]. The fluid allows for smooth movement of the lungs during normal ventilation.

Pleural fluid is absorbed and drained via parietal lymphatic vessels predominantly in the region of the mediastinum and diaphragm where lymphatic vessel numbers have been shown to be greatest. These vessels ultimately drain into mediastinal lymph nodes. The visceral pleura and its capillaries and lymphatic vessels, plays an insignificant part in pleural fluid production and turnover [7].

Normal fluid physiology

Pleural fluid volume is maintained by the balance of pulmonary capillary hydrostatic and oncotic pressure, lymphatic drainage and the integrity of the pleural and capillary membranes. Disturbances to any of these can lead to the formation of excess pleural fluid.

Normal pleural fluid is formed from the filtration of plasma by healthy parietal pleural membranes and has the following characteristics:

• a glucose content similar to plasma
• a low sodium content (1-2 g/dl)
• a low white cell count (<1000 cells / mm3)
• a low lactate dehydrogenase (LDH) level (<50% that of plasma 

Figure 1. Pleural anatomy