Large intestine

Large intestine
Front of abdomen, showing the large intestine, with the stomach and small intestine in dashed outline.
Front of abdomen, showing surface markings for liver (red), and the stomach and large intestine (blue).
Latin	intestinum crassum
Gray's	subject #249 1177
Lymph	inferior mesenteric lymph nodes
Dorlands/Elsevier	i_11/12456545

The large intestine is the terminal, tubular portion of the gastrointestinal tract (gut) of vertebrates prior to the anus or cloaca. Although there are huge differences in size and complexity among taxa, in all species the large intestine is involved in three functions: recovery of water and electrolytes (sodium, chloride) from indigestible food matter, formation and storage of feces, and microbial fermentation (Bowen 2006).

The large intestine has many microbial species that produce enzymes capable of digesting many of the molecules that are indigestible to vertebrates. While these microorganisms receive the benefit of nutrients and habitat, they also provide a larger value for the animal as well. For example, undigested polysaccharides are metabolized to short-chain fatty acids and absorbed by passive diffusion, and the intestinal fauna also produce small amounts of vitamins for absorption into the blood. When dietary intake of vitamins is low, this source can be significant. Remarkably, the immune system can distinguish between these helpful bacteria and pathogens.

Overview

The vertebrate intestine—the long tube that extends from the stomach to the anus or cloaca— tends to be divided into a small intestine and a large intestine, with the lower portion designated the large intestine.

While the function of the large intestine remains basically the same—absorbing the remaining water and electrolytes from ingesta, forming, storing and eliminating these unusable food matter (wastes), and microbial fermentation—the size and complexity varies among taxa. Some vertebrate taxa lack a large intestine. For example, killifish (Fundulus heteroclitus) have a simple digestive system lacking both a large intestine and stomach (but possessing a small intestine) (Vetter et al. 1985) and insectivores lack a large intestine (Palaeos 2003). Herbivores like horses and rabbits, which depend on microbial fermentation, tend to have a very large and complex large intestine, while carnivores like cats and dogs tend to have a simple and small large intestine (Bowen 2000). Omnivores like pigs and humans tend to have a substantial large intestine, but smaller and less complex than that of herbivores (Bowen 2000).

Three major portions of the large intestine generally are recognized in mammals: caecum (blind-ended pouch), colon (majority of the length of the intestine), and rectum (short, terminal segment) (Bowen 2000). The colon often is incorrectly used in the meaning of the whole large intestine altogether; it is really only the biggest part of the large intestine.

Although called the large intestine, in mammals this tube is shorter than the small intestine, but is wider.

Function and relation to other organs

In humans, the large intestine starts in the right iliac region of the pelvis, just at or below the right waist. It is joined to the bottom end of the small intestine. On average, the diameter of the large intestine of an adult human measures about 7.6 centimeters in diameter, while the small intestine measures approximately 2.5 to three centimeters.

The large intestine in humans is typically four to five times shorter than the small intestine. The large intestine is about 1.5 meters long, which is about one-fifth of the whole length of the intestinal canal. In humans over five years old, the small intestine is about seven meters (23 ft) long.

The large intestine of humans 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 three 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 MALT that 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

Bacterial flora

The large intestine of humans 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 neutralize the increased acidity resulting from the formation of these fatty acids.

These bacteria also produce small amounts of vitamins, especially vitamin K and the vitamin B group, 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 inflammable 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.

Bacteria also are involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, which also are 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 in humans are:

• Caecum or cecum - the first part of the large intestine, low on the right side.
• Taeniae coli - three bands of smooth muscle.
• Haustra - bulges caused by contraction of taeniae coli.
• Epiploic appendages - small fat accumulations on the viscera.
• Colon - ascending, then transverse, then descending parts of the colon.
• Rectum—low on the left side; final straight portion of the large intestine in some mammals, and the gut in others.

The main function of the colon appears to be extraction of water from feces. In mammals, it consists of the ascending colon, transverse colon, the descending colon, and the sigmoid colon. The colon from cecum to the mid-transverse colon is also known as the right colon. The remainder is known as the left colon.

Additional images

• 

  Sigmoid colon and rectum, showing distribution of branches of inferior mesenteric artery and their anastomoses.

References

ISBN links support NWE through referral fees

• Bowen, R. 2006. The large intestine: Introduction and index. Colorado State. Retrieved July 1, 2007.
• Bowen, R. 2000. Gross and microscopic anatomy of the large intestine. Colorado State. Retrieved July 1, 2007.
• Palaeos. 2003. Insectivora. Palaeos. Retrieved July 1, 2007.
• Vetter, R. D., M. C. Carey, and J. S. Patton. 1985. Coassimilation of dietary fat and benzo(a)pyrene in the small intestine: An absorption model using the killifish. Journal of Lipid Research 26: 428-434.

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