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Blood clots. Coloured scanning electron micrograph (SEM) of small blood clots (red) in a blood vessel. The clots consist of red blood cells, white blood cells, and platelets, all trapped in a mesh of fibrin. The clotting process is started by the platelets. A blood clot forms naturally in response to damage to a blood vessel, but abnormal blood clots can cause strokes and heart attacks.
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