Blood cells, coloured scanning electron micrograph (SEM). Seen here are slightly crenated (wrinkled) red blood cells (erythrocytes) and a single white blood cell (yellow). The main function of red blood cells is to distribute oxygen to body tissues, and to carry waste carbon dioxide back to the lungs. White blood cells are part of the immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. Magnification: x1000 when printed at 10 centimetres high.