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. These are the thinner vessels (capillaries) of the inner wall, which branch off from main vessels (not seen). 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.