A colon that can stretch from 80 centimetres to over three metres
The large intestine is the final stretch of the gut in four-limbed animals, where water and salts are pulled back from waste and faeces wait in the rectum. It is called large for its width, not its length: in people it runs about 1.5 metres, a fifth of the digestive tract.
In humans it begins low on the right side of the abdomen, where the small intestine empties through the ileocecal valve into the cecum. From there the colon climbs, crosses the abdomen, descends and ends at the rectum and anal canal. Doctors often use colon and large intestine as if they were the same, but most definitions count the cecum, colon, rectum and anal canal together, and some leave the anal canal out.
The colon is by far the longest part. Adult men average 166 cm, with a range of 80 to 313 cm, and women 155 cm, ranging from 80 to 214 cm. Its four sections, ascending, transverse, descending and sigmoid, meet at bends called colic flexures, and three bands of smooth muscle, the taeniae coli, are shorter than the bowel itself, which bunches it into the pouches known as haustra. The cecum is the widest segment at just under 9 cm, while the sigmoid colon averages 4 to 5 cm; unusually wide sections can signal megacolon.
Some segments are anchored behind the abdominal lining while others, like the transverse colon, hang freely in it, which affects how easily surgeons can reach them. The transverse colon draws blood from two arteries, and the boundary between them is vulnerable to loss of blood supply. About 1.5 litres of water enters the colon daily, and gut microbes ferment what the small intestine left behind.
The appendix grows from the cecum but plays no part in digestion. It belongs to the gut’s lymphoid tissue, and some researchers think it keeps a reserve of gut bacteria that can restock the colon after illness. The muscular sigmoid colon squeezes stool onward into the rectum, which holds it until defecation.
Source: Large intestine
