Several years ago the deaths of several airline travelers led to warnings about “economy-class syndrome.” The idea is that economy-class passengers don’t have as much leg room as passengers in more expensive seats, so they are more likely to sit essentially motionless for long periods on flights—conditions that may allow blood to pool and clots to form in the legs. This condition is called deep-vein thrombosis, or DVT. Given what you know about blood flow in the veins, explain why periodically getting up and moving around in the plane’s cabin during a long flight may lower the risk that a clot will form.