Blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, red) and a platelet (thrombocyte, yellow). Some of the erythrocytes are crenated (spiked). They have dehydrated and distorted in shape. Erythrocytes are carriers of oxygen and carbon dioxide. Their biconcave surface provides maximum surface area for the exchange of gases. Platelets are fragments of white blood cells. When blood clots, red blood cells are trapped within a fibrin protein mesh, which is formed in response to chemicals secreted by the platelets. Magnification: x2600 when printed 10cm wide.