BY Jennifer Hong Edema is a sign, not a disease. It is defined as the abnormal movement of substances between the plasma and interstitial fluid across the capillary which results in an increase of interstitial fluid volume. Hydrostatic pressure is the pressure of water in the fluid against the blood vessel walls. This gradient changes from the arteriole end of the capillary to the venous end.
Osmotic pressure is determined by plasma protein concentration.
Since the capillary is impermeable to plasma proteins, their relative concentrations between the plasma and interstitial tissues are constant. Thus, osmotic pressure is held constant from the arteriole to the venous end of the capillary. At the arteriole end, hydrostatic pressure is high and thus promotes fluid filtration. At the venous end, hydrostatic pressure drops below that of osmotic pressure. Hence, the result is fluid reabsorption.