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In medicine, an embolism occurs when an object (the embolus, plural emboli) migrates from one part of the body (through circulation) and cause(s) a blockage (occlusion) of a blood vessel in another part of the body. This can be contrasted with a "thrombus" which is the formation of a clot within a blood vessel, rather than being carried from elsewhere. Blood clots form the most common embolic material by far: other possible embolic materials include fat globules (a fat embolism), air bubbles (an air embolism), septic emboli (containing pus and bacteria), or amniotic fluid. Emboli often have more serious consequences when they occur in the so-called "end-circulation": areas of the body that have no redundant blood supply, such as the brain, heart, and lungs. Assuming a normal circulation, a thrombus or other embolus formed in a systemic vein will always impact in the lungs, after passing through the right side of the heart. This forms a pulmonary embolism that can be a complication of deep-vein thrombosis. Note that, contrary to popular belief, the most common site of origin of pulmonary emboli is the ''femoral'' veins, not the deep veins of the calf. Deep veins of the calf are the most common site of thrombi, not emboli orgin. (However, when an examination is done soon after embolization, you may find thrombus in the calf because the previous femoral vein thrombus has floated away!) Some congenital abnormalities of the circulation, especially septal defects (holes in the cardiac septum), allow embolus from a systemic vein to cross into the arterial system and land anywhere in the body. The most common such abnormality is patent foramen ovale, occurring in about 25 % of the adult population, but here the defect functions as a valve which is normally closed, because pressure is slightly higher in the left side of the heart. In unfortunate circumstances, e.g. if patient is coughing just when an embolus is passing, passage to the arterial system may occur. Emboli starting in the heart (from a thrombus in the left atrium secondary to atrial fibrillation or septic emboli from endocarditis) can cause emboli in any part of the body. An embolus landing in the brain from either the heart or a carotid artery will likely cause an ischemic stroke.
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