Blood platelet, coloured scanning electron micrograph (SEM). Platelets (thrombocytes) are part of the blood. When a blood vessel is damaged, the platelets become activated and secrete chemicals that cause the formation of a fibrin mesh. This mesh traps platelets and red and white blood cells, forming a clot that seals the damaged blood vessel. Magnification: x11,000 when printed at 10 centimetres across.