Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels from the large intestine, or colon. 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 main function of the colon is to reabsorb water from the faeces and then expel it from the body. The cast was made by injecting resin into the blood vessels and then chemically digesting the surrounding tissues.