Pathogenesis of edema is in depth considered based on Starling's rule on capillary fluid dynamics. Edema can be divided into 4 types based on the mechanisms causing edema: increased capillary hydrostatic pressure, decreased plasma oncotic pressure, enhanced hydraulic permeability of capillary walls, and lymphatic obstruction. Each of the types can be further divided into generalized and local forms. Edema, defined as the expansion of interstitial fluid compartment, is initiated by imbalance of Starling forces across capillary walls between plasma and interstitium, but is maintained by enhanced sodium reabsorption from renal tubules.