A Damaged VeinIf heredity, an injury, or a blood clot weakens a vein, the wall near the valve begins to sag. The valve may no longer close fully, allowing blood to move backward. Most vein problems begin with damaged veins. | | A “Ropy” VeinOnce a vein is damaged, blood pressing against the sagging wall may cause the vein to bulge or twist like a rope. Eventually, the valve can’t close. Blood may begin to pool or clot in the vein. | | Pooling BloodA valve that doesn’t close cannot hold blood. Blood moves backward. It pools around the first healthy valve that stops the blood from moving backward. | | Clotting BloodWhen blood moves slowly, it may collect above a valve. Over time, the blood forms a clot, which may grow big enough to close off the vein. |