Blood will pool in the legs for several reasons. The first can be due to the lack of movement (such as standing still for a long time); the gravity prevents the blood from travelling up to the heart due to the lower pressure system. The second may be due to damaged vein valves. The valves that stop the blood from backflowing can be damaged by excessive pooling of blood from lifestyle factors such as standing and crossing your legs. This excessive pooling can cause the surface veins in your legs to bulge with blood and cause varicose veins. The last reason may be due to genetic reasons for weaker walls of the veins.
