6 Things to Know About Varicose Veins About 20 percent of adults have varicose veins, and most everyone has a spider vein or two. While you may not like the way they look, these bluish, lumpy intrusions are usually harmless. However, for some people, they can be painful and dangerous. 1. All leg veins are not the same. Veins are thin-walled vascular structures with valves that keep blood flowing in one direction: toward the heart. Varicose veins form when valves deteriorate, allowing blood to flow downhill and pool in the leg. “Veins can lose their elasticity, and when they lose their elasticity, the...