Veins in the legs carry blood uphill back toward the heart. One-way valves snap shut behind each slug of blood so it cannot drop back down between beats.
Leg muscles act as a second pump. Walking squeezes the veins, and the valves turn that squeeze into upward flow. Standing still removes the help.
The design is mechanical. If the valves weaken, blood can pool in the legs — one reason moving beats standing frozen for circulation.
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