Both veins and arteries are part of our circulatory system. Arteries carry blood rich in oxygen under high pressure away from the heart to the tissues. Poor circulation usually refers to a blockage of these vessels. Vascular surgeons are for the most part trained to deal with problems with arteries. After the blood gives up its oxygen in the tissues, it is routed back to the heart through the veins. Phlebologists specialize in problems of the veins.
The function of veins is to bring blood back to the heart. Most of the arterial blood pressure is lost as the blood travels through the tissues. Returning to the heart can be a difficult process in the legs of an upright person as the venous blood must travel upward against gravity. This is accomplished because veins unlike arteries have one-way valves which keep the blood from flowing back down into the leg.
When these valves don’t function properly the blood flows backwards under increased pressure from gravity, causing the veins to dilate and elongate. These elongated and dilated veins are commonly called varicose veins or spider veins, depending on their location and size.