Edema is the swelling of tissues that appear when excessive fluid accumulates within the tissue. Fluid comprised of water and electrolytes, with a very small aggregate of protein and other macromolecules, normally leaves capillaries and cramped postcapillary venules by a process called filtration. Filtration is primarily consumed by the capillary hydrostatic pressure, and the amount filtered per unit time is additionally influenced by the permeability of the vessel wall. The fluid that penetrates into the tissue discharge in the intercellular space and most of it is reabsorbed at the venular end of capillaries where the hydrostatic pressure is lower. Some of the filtered fluid is taken up by lymphatic vessels and exchanged to the circulation.