Clinical meaning
Edema is the accumulation of excess interstitial fluid caused by an imbalance in the Starling forces governing fluid movement across the capillary membrane. The four Starling forces are: (1) CAPILLARY HYDROSTATIC PRESSURE (pushes fluid OUT of the capillary into the interstitium -- elevated in heart failure, venous insufficiency, and fluid overload), (2) INTERSTITIAL HYDROSTATIC PRESSURE (pushes fluid INTO the capillary -- usually negligible in health), (3) PLASMA ONCOTIC PRESSURE (pulls fluid INTO the capillary -- maintained by albumin; reduced in nephrotic syndrome, liver failure, and malnutrition), and (4) INTERSTITIAL ONCOTIC PRESSURE (pulls fluid OUT into the interstitium -- elevated when capillary permeability increases, as in inflammation, burns, and sepsis). Additionally, LYMPHATIC OBSTRUCTION (from surgery, radiation, malignancy, or filariasis) impairs interstitial fluid drainage, causing lymphedema. SODIUM AND WATER RETENTION by the kidneys (RAAS activation, ADH secretion) compounds edema formation by expanding intravascular volume and increasing capillary hydrostatic pressure. CLINICAL GRADING uses a standardized pitting edema scale: Grade 1+ (2 mm depth, immediate rebound), Grade 2+ (4 mm depth, rebound within 15 seconds), Grade 3+ (6 mm depth, rebound within 30 seconds),...