Pulmonary embolism is blockage of blood flow through a pulmonary artery by a blood clot (embolus). Common in bedridden patients (and airline passengers), an embolus may form in the legs, break away, and because vessels widen towards the heart, be carried in blood only to become lodged in narrowing pulmonary arterioles. Accompanying symptoms depend on the amount of blood vessel obstructed. Sudden unexplained shortness of breath is a hallmark of the disease. Approximately 10% of pulmonary emboli are fatal.