Harvard Heart Letter

Blood clots: The good, the bad, and the deadly

Those arising from atherosclerosis and atrial fibrillation can be very dangerous.

When you poke yourself on a thorn while gardening or get a paper cut at the office, your body marshals the forces needed to stop the flow of blood and repair the damage. If it weren't for the blood's ability to clot (form a thrombus, in medicalese), even these minor scrapes of daily living could cause us to bleed uncontrollably. These healing clots also form inside the body at sites of blood vessel injuries. Normally, when the clot's job is done, it dissolves away.

But sometimes clots form in places where they do more harm than good — like in the arteries that supply the heart or the brain, or in the veins of the legs. Clots inside the body can also be quite mobile, traveling through the bloodstream from place to place. For example, a clot formed in a leg vein can travel to the lungs, and one arising in the heart can end up in the brain. Such silent migrations can have deadly consequences.

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