Venous skin ulcers are caused by poor blood circulation, as a result of venous insufficiency. Your veins have one-way valves that keep blood flowing toward the heart. In venous insufficiency, the valves are damaged, and blood backs up and pools in the veins. When this happens, it is possible that the blood may leak out of the vein and into the surrounding tissue. This can result in the breakdown of the tissue and an ulcer. Veins that become blocked also may cause blood to pool, leading to these ulcers. There are a number of things that can increase your risk of venous skin ulcers. These include: