Large intestine, coloured computed tomography (CT) scan. The large intestine constitutes the last part of the digestive tract. It starts at the at the caecum (centre left) at the end of the small intestine and ends at the rectum (bottom left), which stores the faeces before it is expelled from the anus. The main function of the large intestine is to reabsorb water from the faeces and then expel it from the body. Its convoluted shape gives a large surface area for reabsorption. CT scans are produced using multiple X-rays collated by a computer. A radio opaque contrast medium has been injected into the large intestine to highlight it on this X-ray.