The colon is the last part of the digestive system, and together with the cecum and rectum makes up the large intestine. Although it is a part of the digestive system, the colon does not actually carry out any digestive functions; indeed, the entire big intestine is not involved in any of the digestive work that goes on in the body, the entire chemical digestion of food having been completed in the small intestine.
The colon is made up of four parts. The ascending colon is the part of the colon that lies on the right side of the abdomen and runs from the point at which the colon attaches to the cecum to the hepatic flexure, i.e. the point at which the colon makes a turn beside the liver. The transverse colon is that part of the organ starting from the hepatic flexure and continuing to the splenic flexure, i.e. that point at which the organ makes another turn just by the spleen. This section of the colon is attached to the stomach by a tissue band known as the greater omemtum, and to the back abdominal wall by a serous membrane known as a mesentery. The next section is the descending colon which starts off from the splenic flexure and goes on to the beginning of the sigmoid colon. The descending colon acts as a store for food that will eventually be emptied into the rectum. The final section of the colon is the sigmoid colon, so-called because of its s-shape. The walls of the sigmoid colon are muscular and contractile so that it is able to exert the necessary pressure that will enable stool to be moved from the colon into the rectum.
The large intestine, of which the colon is an integral part, is about 1.5 meters long and its primary function is to store waste and reclaim water thus enabling the body’s water balance to be maintained. As we have noted above, no digestive process goes on in the big intestine and it does not play a major role in the absorption of foods and nutrients. This is not to say, however, that no absorption takes place in the large intestine. Certain essential vitamins such as vitamin K are absorbed here.
The process is as follows. First, the food that we consume is reduced to a pulp, known as chyme, in the stomach before it proceeds to the intestines. As the chyme proceeds through the small intestine, most of its constituent nutrients and water are absorbed into the body. By the time that the chyme enters into the colon, only about ten percent of the original water content is left in the chyme, along with some indigestible dietary fibers, carbohydrates and proteins, as well as electrolytes like chloride, magnesium and sodium. It is as the chyme moves through the colon that most of the remaining water is extracted and the chyme is mixed with mucus and bacteria (gut flora) which causes fermentation of the chyme and turns it into feces.
As the feces, still in liquid form, moves through the ascending colon, the remainder of the water is extracted and, by the time that it is passing on into the descending colon it has become semi-solid. At this point, bacteria once again come into play, for the semi-solid sludge still contains undigested fibers. The bacteria consume some ten or so percent of the remaining fibers (this percentage is higher in animals, e.g. apes, which have a proportionately larger colon) and the waste that these bacteria produce, e.g. acetate, propionate and butyrate, serve in turn to provide nourishment for the cell lining of the colon. This symbiotic relationship is beneficial to both parties, the bacteria getting the nourishment that it needs, and the body getting about a hundred calories of energy per day.