Both spider and varicose veins arise from an underlying condition called venous insufficiency. Veins are responsible for transporting blood from your tissues back to your lungs in order to restore oxygen–the blood travels in one direction, which is ensured by valves that prevent blood from going backward. In venous insufficiency, these valves stop functioning properly and allow blood to pool in the venous structures. In the smaller, more superficial vasculature, this results in spider veins. Larger, bulging veins are called varicose veins.
Spider veins can be treated with the process of sclerotherapy, where a chemical agent is injected into the vein and effectively closes it off, or phlebectomy, in which case the veins are physically removed from below the skin. For varicose veins, chemical and radiofrequency (heat) ablations exist, which function as another way to shut down the vein. Although the vein is closed off, blood is still able to travel through other smaller structures so that blood flow is uninterrupted.