Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels from the small intestine. This network of vessels infiltrates the tissue, supplying it with blood. At bottom are the larger vessels of the outer wall and at top are the thinner vessels of the inner wall. The inner wall contains folds, called villi, that greatly increase the surface area for the absorption of nutrients. The cast was made by injecting resin into the blood vessels and then chemically digesting the surrounding tissues.