Venous ulcers typically occur along the lower leg. They form when venous valves stop functioning efficiently, or when veins become damaged by disease, trauma, or blood clots. The damaged vein can’t help the heart send blood back to it effectively, so the blood begins to pool at the lower extremities. As the vein swells to accommodate the extra blood, it begins to leech into the surrounding tissue, damaging it. This is called edema.