Correct answer:

All the major leg veins have valves that prevent blood flowing away from the heart. As the calf muscles contract, the deep veins within them are squeezed and emptied, the blood passing upwards, directed towards the heart by non-return valves. As the muscles relax, blood flows in from the superficial system via perforators as well as from more distal segments of the vein, only to be pumped upwards again by the next contraction of the calf muscles, which are thus acting as a pump.