Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels supplying the stomach. This branching network of vessels infiltrates the tissue, 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. The surrounding tissues were then chemically digested. Magnification unknown.