Pleural Effusion
Fluid accumulates in the pleural space when the normal balance between fluid formation and pleural lymphatic drainage is disrupted, and whether that imbalance is from raised hydrostatic/low oncotic pressure (transudate) or local pleural or lung pathology (exudate) determines both the underlying cause and the diagnostic pathway.
First principles
A thin fluid film becomes an effusion when formation outpaces drainage
The pleural space normally contains only a thin lubricating film of fluid, continuously formed by the parietal pleura's systemic capillaries and reabsorbed by pleural lymphatics. An effusion develops when this balance tips: either more fluid is formed than the lymphatics can clear, or the lymphatics themselves are overwhelmed or obstructed. Because the mechanisms that tip this balance differ (systemic hydrostatic/oncotic pressure changes versus local pleural or lung inflammation), the resulting fluid has characteristically different composition, which is exploited diagnostically.
Educational content pending clinical review. Not medical advice.