Blood clot, coloured scanning electron micrograph (SEM). Red blood cells (erythrocytes) and a white blood cell (purple) trapped within a fibrin protein mesh (cream). The fibrin mesh is formed in response to chemicals secreted by platelets (brown), fragments of white blood cells. Clots are formed in response to cardiovascular disease or injuries to blood vessels. Magnification: x5,500 when printed at 10 centimetres wide.