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How Veins Work
When the blood reaches the feet it has travelled easily downhill pumped by the powerful action of the heart from above, aided by the elastic recoil of the artery walls and of course, by the effect of gravity! The blood has got to get back again and when you are lying down this is not too much of a problem as the next lot of blood coming in to the feet pushes it onwards along the veins. If you are resting with your feet up gravity plays a big part (Well have you ever seen a waterfall flowing upwards?)

In the standing position, the blood has to be actively pumped up the column of veins. Fortunately, this is achieved with our calf muscles which squeeze the venous sinuses deeply situated within the leg when we walk. This forces the blood upwards into the pelvis. 

However, when the muscles relax, the blood tends to rush back into the leg. This is called reflux. To prevent all the effort being wasted, there are valves which snap shut holding the column of blood until the next calf muscle contraction occurs. This can be heard with the Hand Held Doppler. When the calf is squeezed, a "whoosh" is heard indicating the blood rushing up the veins into the pelvis and out of the leg. In the normal situation, this is followed by an abrupt silence as the valves close thus stopping the flow.