Usually, unobstructed arteries can send enough blood to the affected part of the lung to prevent tissue death. However, when very large vessels are blocked or the person has a preexisting lung disease, the amount of blood supplied may be insufficient to prevent tissue death. About 10 percent of people with pulmonary embolism suffer some lung tissue death, called pulmonary infarction. If the body breaks up small clots quickly, damage is kept to a minimum.
Large clots take much longer to disintegrate, so more damage is done.
Large clots may cause sudden death. Causes The most common type of pulmonary embolus is a blood clot, usually one that begins in a leg or pelvic vein. When the person starts moving again, the clot can break loose. Far less often, clots begin in the veins of the arms or in the right side of the heart. Once a clot in a vein breaks free into the bloodstream, it usually travels to the lungs.