Coloured scanning electron micrograph (SEM) of a resin cast of capillary blood vessels. Capillaries are the smallest blood vessels in the body, only a few micrometres across, just large enough for the passage of red blood cells. They form a fine network in most organs and tissues of the body. Gaseous exchange, where oxygen is delivered to the tissues and carbon dioxide removed, occurs in the capillaries. The cast was made by injecting the capillaries with a two-component resin and then chemically digesting the surrounding tissues. This has left the cell boundaries and the imprints of the cell nuclei of the endothelium visible. The picture was created as part of a cooperation with Quintessenz Publishing. Magnification: x600 when printed at 15cm wide.