Edema, ih DEE muh, is the swelling of body tissues caused by fluid build-up between cells. Cells are bathed and nourished by fluid that continually seeps from the capillaries, the tiniest blood vessels. Normally, the seepage is balanced by reabsorption of fluid into the capillaries and by drainage into the lymphatic system (see Lymphatic system). When this balance is upset, edema results.