Edema (also spelled oedema, formerly known as dropsy) is swelling due to accumulation of excess fluid in any biological tissue. Edema has many root causes, but the mechanism is simple; fluid is drawn from the blood into the tissues when there is a higher osmotic pressure in the tissues than in the blood. (Blood normally has a higher osmotic pressure than the tissues due to the contribution of the oncotic pressure). This higher pressure may be due to an actual increase (e.g., salt retention due to kidney failure) or it may be a relative increase (e.g., edema due to low serum protein in the blood due to nutritional deficiency). Obstruction to venous blood flow also results in edema due to the mechanically caused increase in blood pressure in upstream capillaries. Capillary damage due to infection, bacterial toxins, or other trauma will also allow fluids to move from the blood into tissues, and the exudation of fluid into extracellular spaces is part of the general process of inflammation.
Common conditions causing or characterized by edema are congestive heart failure, some renal problems, varicose veins, cirrhosis, malnutrition and allergic conditions such as angioneurotic edema.