Varicose veins are enlarged and twisted rope-like veins that appear near the surface of the skin. While they can develop anywhere in the body, they are most commonly found in the legs and ankles because standing and walking increase pressure in the lower extremities. In normally functioning veins, tiny one-way valves open as blood flows toward the heart and close to prevent blood from flowing backward. When these valves malfunction, blood pools in the veins, resulting in a buildup of pressure that weakens their walls and causes them to bulge. Over time, the increased pressure can cause additional valves to fail. This venous reflux, or venous insufficiency, leads to the development of varicose veins and spider veins.