Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in the penis. The network of fine vessels seen branching off from the blood vessel at centre infiltrates the tissue of the penis, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissue through the permeable walls of capillaries, the smallest blood vessels. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissues. Magnification: x36 at 6x7cm size.