Veins are hollow, flexible tubes with flaps inside called valves that open and close, directing the blood flow in one direction towards your heart. Venous disease occurs when these valves become weakened or damaged, causing blood to flow down in a retrograde (backward) fashion into the legs and pool in the veins. In simple terms, venous disease is a condition caused by malfunctioning of the valves in the veins of the legs.
With increasing reflux, a vein becomes progressively distended, tortuous, and therefore more visible. This process also makes reflux worse and as the vein increases in diameter it becomes varicosed.