An edema is a swelling of tissue due to an abnormal accumulation of liquid, generally blood serum, a liquid compound found in blood. This phenomenon is caused by plasma
leaking out of blood vessels into the interstitium, the supportive tissue located between the organs. It can be secondary to different processes, but in the majority of cases it is due to an imbalance between different types of pressure applied by the molecules composing biological liquids. The causes for these molecules multiplying, and possibilities of imbalance, can be many different pathologies, leading to an edema. There can be just one organ involved, such as a lung for example, or it can be a generalized process, such as with certain severe allergic reactions
. The sub-cutaneous tissue is often affected, taking on a swollen appearance, with the sensation of pushing into a water-filled object when pressed, with the water slowly flowing back when released.