Veins are pretty important. After all, they carry blood from various parts of the body to the heart. But for all their significance, veins are modest, too. Unless you're a body builder with veins popping out all over, they're usually content to work behind the scenes, tucked away beneath the skin.

However, sometimes veins take center stage and become visible as blue lines on the skin, often resembling an unmarked road map. If you haven't guessed by now, we're talking about varicose veins. What are they, and how do they form?

Basically, a varicose vein is one that is twisted and filled with backed-up blood. Although they most commonly occur in the legs, varicose veins can also happen in the esophagus, the veins that carry blood from the testes, and the veins of the urinary bladder.

Varicose veins in the legs occur when the valves in a vein near the surface of the skin malfunction. Valves usually keep blood from flowing backwards in the vein once it's been pumped towards the heart. When the valves don't work properly, blood backs up in the vein, causing it to swell and twist. Although varicose veins are usually associated with senior citizens, not every older person has them. Weak valves are often inherited, and can also be caused by wearing clothing that squeeze the legs and veins too tightly.

Varicose veins may seem unsightly, but they're not usually a cause for major concern. If you have them and want to do something about it, treatments range from wearing strong support stockings to surgery.