What pathophysiologic process underlies a transudative pleural effusion?

• A. Bleeding into the pleural space
• B. Infection in the pleural space
• C. Decreased lymphatic clearance of fluid from the pleural space
• D. Increased hydrostatic pressure causing fluid leakage

Answer: (D)

Transudative pleural effusions arise from fluid movement driven by pressure gradients across the pleural capillaries rather than inflammation. When hydrostatic pressure inside the capillaries increases—such as in congestive heart failure or volume overload—more fluid leaks into the pleural space. The result is a fluid that is low in protein and LDH, reflecting a filtrate rather than inflammatory exudate. This hydraulic-based leakage contrasts with exudates, which come from inflammation that increases vascular permeability and yields protein-rich fluid. Other mechanisms like bleeding, infection, or lymphatic obstruction produce different types of effusions and aren’t characteristic of transudates.