Veins rely on internal valves to keep blood flowing upward, against gravity.
People who must regularly sit or stand for long periods, or who are bedridden or obese, are all subject to venous insufficiency. Support hose counteract the problem by compressing the veins near the surface, forcing more blood into the larger, deep veins. Blood pressure and velocity of blood flow in these veins are thereby increased, which, in turn, helps push blood back up the length of the leg, to the heart. The compression also helps keep excess fluid from seeping out of small blood vessels into leg tissues, which accordingly swell. To have medically therapeutic effects, the hose must be expertly tailored to exert graded pressure, with the greatest compression at the ankle, gradually diminishing up the length of the leg.