(HealthScoutNews) -- Ceremonial military guards, at places such as Arlington National Cemetery or Buckingham Palace, aren't always as "motionless" as their reputations suggest. Every so often, two guards will turn sharply, march toward and past each other, then resume a fixed stance where their counterpart stood a short while earlier.

There's a reason: Unless the legs are exercised, their muscles can't help push blood upward through the veins and toward the heart. If a soldier stands stiffly for too long -- more than 15 minutes is the rule of thumb -- the blood flow from veins is so affected that he could faint.

That's so because blood in your veins is under low pressure -- so low that veins in your legs have special one-way valves that can only be opened from the bottom, so gravity-influenced blood from the upper body doesn't drain downward and accumulate in the legs or feet.

Arteries, which carry blood away from the heart, have a significantly different structure than veins, in part because they are designed to transport the blood under much higher pressure than veins do.

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