Edema

Edema is the build-up of too much fluid in the interstitial space, the gaps between cells, or in a body cavity. It shows up as swelling, most often in the feet, ankles and lower legs, and it can affect one area (localized) or the whole body (generalized). When pressing on a swollen area leaves a dent that slowly fills back in, it is called pitting edema.

Normally, blood pressure pushes fluid out of the capillaries while the osmotic pull of plasma proteins such as albumin draws most of it back, and lymphatic vessels carry away the small surplus. Edema appears when this balance fails: when pressure inside capillaries rises, as in heart failure or when blood pools in the legs; when plasma protein falls, as in liver disease, some kidney diseases or severe malnutrition; when capillaries become leaky, as in inflammation; or when lymph drainage is blocked.

Edema is a sign of an underlying process rather than a disease in itself. The puffy swelling around an injury comes from inflammation, whereas fluid that collects in the lungs, called pulmonary edema, makes breathing hard. Some medicines, including corticosteroids, can also cause the body to retain fluid.