If somebody spends a lot of time on their feet, or sitting down the valves are have to work very hard to push the blood against the force of gravity. (compressioncarl.com)
The veins have to work against the force of gravity, which makes them tired. (compressioncarl.com)
These valves have an extremely important role to play inside the legs, since they have to constantly fight against the force of gravity to push the blood back towards the heart. (compressioncarl.com)