What distinguishes transudative effusions from exudative effusions in pathology?

Transudative effusions and exudative effusions are distinguished primarily by their causes and the composition of the fluid.

Transudative effusions typically result from systemic conditions that alter the balance of fluid production and absorption in the pleural space, such as congestive heart failure or cirrhosis. These conditions do not involve local inflammation. The fluid in transudates is generally low in protein content and has low specific gravity.

In contrast, exudative effusions arise from local pathological processes, such as infections, malignancies, or inflammatory diseases. These conditions usually involve inflammation, which increases the permeability of the pleural capillaries, allowing proteins and other substances to leak into the pleural space. As a result, exudates have a higher protein concentration and specific gravity.

This distinction based on causes and fluid composition is critical in clinical practice as it helps guide diagnosis and management of the underlying conditions. The other options such as volume, onset time, and color are not reliable indicators for differentiating between transudative and exudative effusions.