The main function of the colon – or large intestine – is to concentrate previously digested material by absorbing water and salt before passing it into the rectum, where is it evacuated.

Billions of bacteria reside in the colon and produce hydrogen, carbon dioxide, hydrogen sulphide and methane – that’s quite a chemical cocktail!

Mucus secreted by the lining of the colon contains antibodies that protect the colon from disease.

After all possible nutrients have been absorbed through the walls of the small intestine, a watery mix of undigested material remains. This is known as chyme.

This mixture of water, bacteria, and fibre pass through the ileocecal valve to the large intestine or colon. The ileocecal valve is located by your right hipbone and separates the contents of the small and large intestine.

The appendix is a small, fingerlike sac that extends off the beginning of the colon. Until recently, the appendix was widely considered useless. Now we know it contains a great deal of lymphatic tissue and plays a role in the storage of probiotics.

However, when the appendix collects bacteria and undigested food, it can swell and may even burst, which is a life threatening situation. People who consume a high fibre diet are much less likely to develop appendicitis. The reason being that fibre keeps the food moving through the intestines, and this regular, fairly rapid transit prevents the accumulation of trapped food residue in the colon.

Besides fibre, feces also contain large quantities of living and dead bacteria, along with debris shed by the lining of the intestines. People who consume highly refined, low-fibre foods may have little actual fibre residue in the faeces. Those who eat substantial quantities of vegetables and bread will have much more residue.

The colon is shorter – only 1-1.5 metres long – and fatter than the small intestine. Its job is to absorb water and remaining nutrients from the chyme and form stool. Up to 10 litres of water pass through the colon each day, two thirds of which come from body fluids. The efficient colon pulls 80% of the water out of the chyme, which is absorbed into the bloodstream.

The large intestine has three main parts – the ascending colon (up the right side of the body), the transverse colon (straight across the belly under the ribs), and the descending colon (down the left side of the body) to the rectum, where the feces exit the body. Stool begins to form in the transverse colon.

The contents of the colon are propelled through it by coordinated muscular movement known as peristalsis.

If the chyme passes through the colon too quickly, water is not absorbed, causing diarrhea. Stool that sits too long in the colon becomes dry and hard to pass, leading to constipation. About two thirds of stool is composed of water and undigested fibre and food products. The other third is composed of living and dead bacteria.

The large intestine contains trillions of bacteria. Helpful bacteria, called flora, lower the pH of the colon, killing disease-causing microbes. Intestinal flora also produce vitamins B and K, protect us from illness, enhance peristalsis, and make lactase for milk digestion. Probiotic bacteria ferment the dietary fibre, producing short-chained fatty acids – butyric, propionic, acetic, and valerate. Butyric acid is the main fuel of the colonic cells. Low butyric acid levels, or an inability of the colon bacteria to properly metabolise butyric acid, have been associated with ulcerative colitis, colon cancer, active colitis, and inflammatory bowel disease.

When the stool is finally well formed, it gets pushed down into the descending colon and then into the rectum. It is held there until there is sufficient volume to have a bowel movement. Two sphincters – rings of muscle – control bowel movements. When enough faeces have collected, the internal sphincter relaxes and your mind gets the signal that it’s time to relieve yourself. The external sphincter opens when you command it.

Because this is voluntary, you can have the urge for a bowel movement, but wait until it’s convenient. If you ignore the urge, water continues to be absorbed back into the body and the stool becomes dry and hard. Some people are chronically constipated because they don’t want to take the time to have a bowel movement.

Many health problems arise in the lower GI tract – appendicitis, constipation, diarrhea, diverticular disease (diverticulosis and diverticulitis), Crohn’s disease, ulcerative colitis, rectal polyps, colon cancer, irritable bowel syndrome, parasites and hemorrhoids.

Diverticulosis, a condition in which small ballooned sacs of intestinal membrane have been forced outward by high pressure, is one of the most common lower GI tract disorders. Appendicitis, although not a digestive disorder per se, is another problem that arises in this region.

Colon cancer, second in prevalence only to lung cancer when statistics for men and women are combined, is another potentially dangerous condition that arises in the lower GI tract.