Normal intestines. Coloured 3D computed tomography (CT) scan of the abdomen of a 53 year old patient, showing healthy intestines (brown). The intestines start with the small intestine (central narrow tube) which is surrounded by the wider large intestine tube (the far left, far right and upper regions of the intestines), and ends with the rectum (lowest region). The main function of the intestine is to absorb nutrients and water from food, where its convoluted shape gives a large surface area for absorption. Indigestible remains then pass through the rectum as faeces. The patient's skeleton (blue) can also be seen, with the ribs at the top and the pelvis at the bottom.