Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in the small intestine. This network of vessels infiltrates the tissues, supplying them with blood. Gases and nutrients are exchanged between the blood and surrounding tissue through the permeable walls of capillaries, the smallest blood vessels. The small intestine is the site of most of the absorption of nutrients from food. The cast was made by injecting resin into the blood vessels. The surrounding tissues were then chemically digested. Magnification unknown.