Spider veins are red and purple blood vessels that occur in patches on the legs and face. The network of vessels is often as delicate as a spider's web, which is how they get their name, but the patches also resemble tiny sunbursts, branching trees or just plain old lines.
Spider veins and varicose veins are not the same -- and one does not lead to the other. Whereas varicose veins are large, swollen and occur singularly on the legs, spider veins are delicate and tend to be come in cliques. They are, however, caused by similar factors.
Exactly what happens in the body to produce spider veins isn't known for sure, though there are several theories that address possible causes. One theory is that spider veins, which occur near the surface of the skin, are fed by underlying varicose veins too small or embedded too deeply to reach the skin surface. These underlying veins disrupt circulation, causing spider veins to grow above them. Another theory is that spider veins are little arteries that have latched on to the network of veins nearest the skin surface. Because arteries are highly pressurized and the surface veins have low pressure, the combination of the arterial blood (oxygenated blood flowing to the heart) with the venal blood (deoxygenated blood flowing away from the heart) causes spider veins to be visible.
Spider veins do not pose a health threat. Though they can cause aching and discomfort, spider veins are primarily a cosmetic problem.