Veins are blood vessels also known as vessel of capacitance because they contain most of the circulating blood. Veins, in fact, haven’t so many muscle cells on their wall and, therefore, needs some valves inside to allow blood to flow against the gravity from the lower parts of the body to the heart.
When these valves do not work properly, i.e. they are incontinent, the blood has a greater difficulty in returning to the heart pump. In these cases there is a stagnation of blood in sloping areas of the body: it is called venous stasis.
This increases pressure in veins and capillaries (microcirculation) and, as a consequence, increases leakage into the interstitial subcutaneous tissue: edema.