Obstruction of blood flow by an embolusa substance (e.g., a blood clot, a fat globule from a crush injury, or a gas bubble) not normally present in the bloodstream. Obstruction of an artery to the brain may cause stroke. Pulmonary embolism (in the pulmonary artery or a branch) causes difficulty breathing, chest pain, and death of a section of lung tissue, with fever and rapid heartbeat. Embolism in a coronary artery can cause heart attack. See alsothrombosis.