Originally Posted by JohnDThompson

Actually, arteries lack valves and veins have valves. The problem is that as we get older, the valves in our legs -- the most highly stressed venous valves -- become less effective. That, in conjunction with atherosclerotic plaque build-up (particularly a problem with diabetics, who also may suffer neuropathy that prevents them from sensing a problem in their extremities) means that venous stasis and clots are a risk. Activity, as you note, helps prevent venous stasis, but for someone with advanced peripheral vascular disease it may not be enough.