Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes) on the connective tissue surface of a muscle. Some of the red blood cells are crenated (spiked). They have 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: x3000 when printed at 10 centimetres wide.