Large intestine. Coloured computer tomography (CT) scans of sections through a 68 year old's abdomen, showing a healthy large intestine (colon, black). The colon starts at the end of the small intestine and ends at the rectum (bottom right). The main function of the large intestine is to absorb water from the faeces. It has a convoluted shape giving it a large surface area for absorption. Prior to this scan the colon was distended with air in order to highlight any anomalies.