Red blood cells. Coloured scanning electron micrograph (SEM) of two red blood cells (erythrocytes). A normal red blood cell is at left. At right is a crenated (spiked) red blood cell. It has dehydrated and distorted in shape. The main function of red blood cells is to distribute oxygen to body tissues, and to carry waste carbon dioxide back to the lungs. Magnification: x10,000 when printed at 10 centimetres wide.