Images not to be used by the tobacco industry. Red blood cells (erythrocytes) and platelets (thrombocytes) in a fibrin mesh, coloured scanning electron micrograph (SEM). A blood clot is starting to form with many erythrocytes (red) becoming entangled in a fibrin mesh (grey). Erythrocytes transport oxygen from the lungs to the rest of the body and also remove carbon dioxide from the body by transporting it to the lungs where it is then exhaled. When a blood vessel becomes damaged, it will send a signal to the platelets (purple) who respond by travelling to the damaged area and transforming into their active form as seen in this image. Fibrin is a protein formed from fibrinogen during the clotting of blood. The platelets clump together and, along with the polymerised fibrin, form a clot over the wound. Magnification: x1150 when printed at 10cm wide.