Blood vessels. Coloured scanning electron micro- graph (SEM) of a resin cast of blood vessels in the duodenum, part of the small intestine. The fine network of smaller vessels branching off from the main vessels infiltrates the tissue, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissues through the permeable walls of capillaries, the smallest of blood vessels. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissue. Magnification: x25 at 6x7cm size.