False-colour scanning electron micrograph (SEM) of human red blood cells, or erythrocytes. Erythrocytes transport oxygen to the cells & tissues of the body. Mature erythrocytes are biconcave discs about 7.5 microns in diameter. The biconcave shape is an adaptation to increase the surface area over which the exchange of oxygen & carbon dioxide takes place. Erythrocytes are in effect bags of haemoglobin, the blood's oxygen-carrying molecule. They have no cell nucleus. They are highly flexible & able to squeeze through small capillaries. The spikey cells here are crenated red blood cells called echinocytes. Magnification: x1120 at 35mm size.