Small intestine blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in the duodenum region of the small intestine. The thick blood vessels (orange) are the veins, while the thinner blood vessels (red) are the arteries. These bring blood to and from the smallest blood vessels (capillaries) that infiltrate the walls of the intestine, forming the tube shape seen here. It is in the capillaries that nutrients (from digested food) and gases (oxygen for energy and waste carbon dioxide) are exchanged with the intestinal tissues. The cast was made by injecting resin into the blood vessels. The surrounding tissues were then chemically digested.