Large intestine

The large intestine, an organ which is now more commonly referred to by its Greek name, the colon, is the last part of the digestive system: the final stage of the alimentary canal in vertebrate animals. Its function is to absorb the remaining water from indigestible food matter, stores these unusable food matter (wastes) and then eliminates the wastes from the body.

The large intestine starts in the right iliac region of the pelvis, just at or below the right waist. Joined to the bottom end of the small intestine, it consists of the cecum and colon. The large intestine is about 1.5 meters long, which is about one-fifth of the whole length of the intestinal canal.

Function and relation to other organs
The large intestine takes 12 to 25 hours to finish up the remaining processes of the digestive system. Food is not broken down any further in this stage of digestion. The large intestine simply absorbs vitamins that are created by the bacteria inhabiting the colon. It is also very important in absorbing water and compacting the feces.

The large intestine differs most obviously from the small intestine in being wider and in showing the longitudinal layer of the muscularis to have been reduced to 3 strap-like structures known as the taeniae coli. The wall of the large intestine is lined with simple columnar epithelium. Instead of having the evaginations of the small intestine (villi) the large intestine has invaginations (the intestinal glands). While both the small intestine and the large intestine have goblet cells, they are abundant in the large intestine.

The vermiform appendix is attached to its posteromedial surface of the large intestine. It contains masses of lymphoid tissue. It is a part of mucosa-associated lymphoid tissue which gives the appendix an important role in immunity. Appendicitis is the result of a blockage that traps infectious material in the lumen. The appendix can be removed with no damage or consequence to the patient.

The large intestine extends from the ileocecal junction to the anus and is about 1.5m long. On the surface, you can identify bands of longitudinal muscle fibers called taeniae coli, each about 5mm wide. There are three bands and they start at the base of the appendix and extend from the cecum to the rectum. Along the sides of the taeniae, you will find tags of peritoneum filled with fat, called epiploic appendages (or appendices epiploicae). The sacculations, called haustra, are characteristic features of the large intestine, and distinguish it from the rest of the intestinal tract.

Bacterial flora
The large intestine houses over 700 species of bacteria that perform a variety of functions.

The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. The bicarbonate the large intestine secretes helps to neutralise the increased acidity resulting from the formation of these fatty acids.

These bacteria also produce small amounts of vitamins, especially vitamin K and Biotin a B vitamin, for absorption into the blood. Although this source of vitamins generally provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin deficient if treated with antibiotics that inhibit other species of bacteria as well as the disease-causing bacteria.

Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the flammable gases hydrogen, methane, and hydrogen sulphide. Bacterial fermentation of undigested polysaccharides produces these.

The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.

They are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion.

The most prevalent bacteria are the bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.

A mucus layer protects the large intestine from attacks from colonic commensal bacteria. This mucus layer is called the mucosal barrier.

Parts and location
Parts of the large intestine are:
 * Cecum - the first part of the large intestine
 * Taeniae coli - three bands of smooth muscle
 * Haustra - bulges caused by contraction of taeniae coli
 * Epiploic appendages - small fat accumulations on the viscera

Locations along the colon are:
 * The ascending colon
 * The right colic flexure (hepatic)
 * The transverse colon
 * The transverse mesocolon
 * The left colic flexure (splenic)
 * The descending colon
 * The sigmoid colon - the s shaped region of the large intestine