1. Increased Hydrostatic Pressure
One of the most common reasons for edema is when the pressure inside blood vessels, especially the veins, becomes too high. Normally, blood flows smoothly through arteries, capillaries, and veins, with only a small amount of fluid leaking out into tissues.
But when the venous pressure increases, too much fluid is pushed out of the capillaries and accumulates in the tissues. This situation is often seen in heart failure, where the heart is too weak to pump blood effectively, causing blood to back up in the legs and ankles.
Similarly, in deep vein thrombosis (DVT), a blood clot blocks a vein, raising pressure behind the clot and leading to swelling in that leg. People with chronic venous insufficiency—a condition where the valves in leg veins are damaged and do not allow blood to return properly—often notice long-term ankle and leg swelling that worsens after standing for long periods.
Even liver cirrhosis, which increases pressure in the portal vein system, can cause swelling in the abdomen (ascites) due to high hydrostatic pressure.