False-colour scanning electron micrograph (SEM) of red blood cells or erythrocytes (seen as the orange biconvave discs) densely packed into a blood capillary. Capillaries are the smallest type of blood vessel, made up exclusively of endothelial cells. Erythrocytes are essentially non-motile bags of haemoglobin; their biconcave shape is an adaptation to increase the surface area over which the exchange of oxygen & carbon dioxide can occur. They are flexible, & can accommodate themselves to the smallest capillaries. Magnification: x600 at 35mm size.