Phlebitis is defined as the formation of a blood clot (thrombosis) in a vein, which is responsible for obstructing blood flow in that vein. The veins of the lower extremities are more affected than the other veins of the rest of the body, because the blood stagnates there more and the veins are more dilated, which promotes the appearance of the clot. Phlebitis of the lower extremities is a serious disease due to a possible acute complication: pulmonary embolism.