Exudates and transudates

Exudates and transudates are fluid accumulations distinguished by composition and formation: exudates usually reflect inflammation and increased vascular permeability, while transudates usually reflect altered hydrostatic or oncotic pressure.

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Starling forces: Hydrostatic and oncotic pressures that govern fluid movement across capillary walls. Their imbalance produces pressure-driven transudates.
Light's criteria: A biochemical rule set that classifies pleural fluid as exudative or transudative. It compares pleural-fluid and serum protein and lactate dehydrogenase to identify exudates.
Modified transudate: A fluid accumulation with features between conventional transudate and exudate, often associated with chronic pressure-driven disease. It shows why a strict two-category scheme can fail in longstanding effusions.
Capillary: A microscopic blood vessel where gases, nutrients, and fluid exchange with tissues. Capillary walls are the site where pressure and permeability shape fluid movement.
Vascular permeability: The ease with which substances cross blood-vessel walls. Inflammation increases permeability, allowing protein-rich exudate to enter tissues or cavities.
Pleural effusion: An abnormal accumulation of fluid in the pleural space surrounding the lungs. Pleural fluid is routinely classified as exudative or transudative.
Pseudoexudate: A transudative effusion that meets biochemical criteria for an exudate, often after diuretic treatment. Diuresis can concentrate pleural fluid and misclassify a pressure-driven effusion.
Albumin: The most abundant protein in blood plasma, produced mainly by the liver. Its concentration contributes strongly to plasma oncotic pressure and fluid retention.
Hydrostatic pressure: The pressure exerted by fluid against the walls of its containing space. Elevated capillary hydrostatic pressure can drive low-protein fluid into body cavities.
Ascites: An abnormal accumulation of fluid in the peritoneal cavity. Fluid composition and serum–ascites albumin gradient help distinguish its causes.