Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in the intestines. 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. This has formed holes surrounding the villi, the protruding folds found in the intestinal lining. This was part of the ileum, the final portion of the small intestine.