How does the blood of the veins in the legs get to the heart?
The veins in your legs have one way valves. These prevent blood from flowing down, but do not actively push it up. However, normal muscle movement from walking etc will squeeze the veins to push blood through these valves, and so it will work its way up to where negative pressure in your heart atrium can begin having an effect. This does however mean that blood accumulates from long periods of standing without movement, and so exercises are needed to prevent fainting, for example, for soldiers on parade (see Pathophysiology, Homeostasis and Nursing
By Roger Watson, Tonks N. Fawcett, Josephine N. Fawcett)