Blood clot. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes) trapped in a fibrin mesh. The production of fibrin is triggered by cells called platelets, activated when a blood vessel is damaged. The fibrin binds the various blood cells together, forming a solid structure called a blood clot. A blood clot is a normal response, preventing an excessive loss of blood. However, inappropriate clotting (thrombosis, forming a thrombus) is a major cause of heart attacks, embolisms, and strokes. Magnification: x3300 when printed at 10 centimetres across.
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