Arteries carry blood away from the heart, and round the body.
Veins carry blood back to the heart.
Varicose veins are swollen veins on the surface of the leg. They usually start on the calf, but can be found right up to the groin They are more obvious when you stand up. Varicose veins affect humans because we stand upright rather than run around on all fours. This means that gravity puts pressure on the veins in the legs, and they have to work harder to pump blood back upwards to the heart. Sometimes this pressure can break the valves in the veins which should only let the blood go one way - towards the heart.