Large intestine. Double contrast coloured X-ray of the abdomen showing a healthy large intestine, the final section of the digestive tract. It starts at the end of the small intestine and ends at the rectum. The main function of the large intestine is to reabsorb water from the faeces. Its convoluted shape gives a large surface area for reabsorption. A double contrast X-ray is obtained by introducing a solution of radio-opaque barium sulphate to the colon, which is then distended by air.