Colon

The colon - also known as the large intestine - is the final organ of the digestive process. It is responsible for drying out indigestible food residues by absorbing fluid and producing solid waste (faeces) for elimination. Approximately 1.5 metres long, the colon has six distinct regions leading from the join with the small intestine (ileocaecal valve): caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum.

As in the rest of the digestive tract, the colon wall is muscular, which is necessary to move waste along the colon. Propulsion along the colon is slower than the small intestine, movement taking days rather than hours. Movement is stimulated by food and exercise, but is diminished during sleep. Transit time in the colon is about 1cm per hour.

Billions of bacteria inhabit the colon, where they ferment dietary fibre and other unabsorbed substances. The colon acts as a storage organ, where mixing movements promote absorption of water, electrolytes and bacterial fermentation products. Mucus produced in the colon is important in easing the passage of waste products towards the rectum and anus.