Crenated red blood cell. Coloured scanning electron micrograph (SEM) of a crenated (spiked) human red blood cell, known as an echinocyte. A red blood cell is usually biconcave, however this cell has been exposed to a concentrated solution, causing it to shrink. This osmotic shrinkage causes the change of shape. The main function of the red blood cell is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. Magnification: x12,000 when printed 10 centimetres wide.