Leg vein. Power Doppler ultrasound scan of blood flow (orange) through the long saphenous vein in a 65-year-old woman's leg. This vein is one of the two saphenous leg veins (long and short) in each leg, superficial (surface) blood vessels that drain blood all along the leg from the foot to the groin. The long saphenous vein is the longest vein in the body. The venous system drains blood back towards the heart. Doppler ultrasound scanning detects moving fluids, such as blood, by the frequency change they cause when reflecting high- frequency sound waves. Power Doppler scans colour the moving fluid, but do not show the direction of the flow. The vein is about 3 millimetres across.