Large intestine. Coloured barium X-ray of the abdomen showing a healthy large intestine (green), the final section of the digestive tract. Bones are pale blue. The main part of the large intestine is the colon, although its final section, the rectum (lower centre), is also seen. The intestine provides a large surface area for the absorption of water, vitamins and mineral salts, and produces mucus to assist faecal movement and to protect its wall from mechanical damage. This is an air, or double, contrast barium enema. Water containing radio-opaque barium sulphate powder was introduced into the colon, which was then distended by air.