Intestine

In anatomy, the intestine is that tubular portion of the Gastrointestinal tract (alimentary canal or digestive tract) of vertebrates extending from the stomach to the anus or cloaca. The intestine tends to be divided into a small intestine and a large intestine, with the lower portion designated the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum, and ileum, while the large intestine is subdivided into the cecum, colon, and rectum.

Although there are huge differences in size and complexity among taxa, in all species the intestine is involved in four functions: Digestion and absorption of nutrients, recovery of water and electrolytes from indigestible food matter, formation and storage of feces, and microbial fermentation (Bowen 2006). The small intestine generally also has an immune function in protection against invaders.

The human intestine is suited for an omnivorous diet that can digest a great diversity of plant and animal matter, allowing humans to interact with innumerable edible organisms in the environment. It can be considered fairly typical of the vertebrates, with other mammals, birds, reptiles, amphibians, and fish having the same basic organs, but modified for more limited and specialized diets or other ecological limits (Moffatt 2001).

Basic structure and function

The vertebrate intestine is a long tube that extends from the stomach to the anus or cloaca. It tends to be divided into a small intestine and a large intestine. The small intestine is the narrow tube of vertebrates between the stomach and the large intestine that is responsible for most of the digestion. The large intestine is the terminal, tubular portion of the gastrointestinal tract (gut) prior to the anus or cloaca. In all vertebrates, 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).

In mammals, including humans, the small intestine is divided into three sections: The duodenum, jejunum, and ileum. In mammals, three major portions of the large intestine also are generally recognized: 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, it is only wider.

Diversity among vertebrates

There is a great deal of diversity in size and form in the intestine among vertebrates. In cartilaginous fish and some primitive bony fish (eg., lungfish, sturgeon), the intestine is relatively straight and short, and many fish have a spiral valve (Ritchison 2007). Amphibians, reptiles, birds, and mammals, as well as some fish, tend to have an elongated and coiled small intestine (Ritchison 2007). Sharks bridge the need for an intestine that can digest large bulk food (for which a long intestine would normally be beneficial) with the need for a streamlined body (where the abdomen narrows rapidly), by having a short, thick, tapering intestine with a complex interior (Moffatt 2001). A wall through the center creates a passage resembling a spiral staircase, where food passes slowly around the intestine as it move toward the colon (Moffatt 2001).

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).

Structure and function in the human body

As with other vertebrates, the intestinal tract of humans can be broadly divided into two different parts, the small and large intestine. Grayish-purple in color, the small intestine is the first and longest portion of the intestine, measuring 6-8 meters (22-25 feet) on average in an adult man. Shorter and relatively stockier, the large intestine is a dark reddish color, measuring roughly 1.5 meters (5 feet) on average. 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.

Both intestines share a general structure with the whole gut, and is composed of several layers. The lumen is the cavity where digested material passes through and from where nutrients are absorbed. Along the whole length of the gut in the glandular epithelium are goblet cells. These secrete mucus, which lubricates the passage of food along and protects it from digestive enzymes. Villi are vaginations of the mucosa and increase the overall surface area of the intestine, while also containing a lacteal, which is connected to the lymph system and aids in the removal of lipids and tissue fluid from the blood supply. Micro villi are present on the epithelium of a villus and further increase the surface area over which absorption can take place.

The next layer is the muscularis mucosa which is a layer of smooth muscle that aids in the action of continued peristalsis along the gut. The submucosa contains nerves, blood vessels, and elastic fiber with collagen that stretches with increased capacity but maintains the shape of the intestine. Surrounding this is the muscularis externa, which comprises longitudinal and smooth muscle that again helps with continued peristalsis and the movement of digested material out of and along the gut.

Lastly there is the serosa, which is made up of loose connective tissue and coated in mucus so as to prevent friction damage from the intestine rubbing against other tissue. Holding all this in place are the mesenteries, which suspend the intestine in the abdominal cavity and stop it being disturbed when a person is physically active.

The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to breakdown itself. This is an example of symbiosis. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence when removed through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus), the reabsorption of sodium, as well as any nutrients that may have escaped primary digestion in the ileum.

Absorption of glucose in the ileum

Initially, nutrients diffuse passively from the lumen of the ileum via the epithelial cells and into the blood stream. However, certain molecules like glucose passively diffuse in mass quantity some time after a meal, causing a change in concentration gradient. This results in a higher concentration of glucose in the blood (blood sugar level) than in the ileum, such that passive diffusion is no longer possible. Active uptake would be a waste of energy, so another process is used to transport the left-over glucose from the lumen into the blood stream.

In this process, called secondary active transport, a glucose molecule associates with a sodium ion and approaches a transporter protein in the membrane of an epithelial cell. The protein allows the sodium ion through, which then "pulls" the glucose molecule into the cell. Once inside the cell, the sodium and glucose dissociate, and the glucose molecule is free to diffuse passively from the cell into the blood stream (this is because the blood flowing past the cell has a lower blood sugar level than the cell cytoplasm).

Diseases

• Gastroenteritis is inflammation of the intestines and is the most common disease of the intestines. It can arise as the result of food poisoning.
• Ileus is a blockage of the intestines.
• Ileitis is an inflammation of the ileum.
• Colitis is an inflammation of the large intestine.
• Appendicitis is inflammation of the vermiform appendix located at the cecum. This is a potentially fatal disease if left untreated; many cases of appendicitis are treated by surgical intervention.
• Coeliac disease is a common form of malabsorption, affecting up to 1 percent of people of northern European descent. Allergy to gluten proteins, found in wheat, barley, and rye, causes villous atrophy in the small intestine. Life-long dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
• Crohn's disease and ulcerative colitis are examples of inflammatory bowel disease. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.
• Enteroviruses are named by their transmission-route through the intestine (enteric=related to intestine), but their symptoms are not mainly associated with the intestine.

Disorders

• Irritable bowel syndrome is the most common functional disorder of the intestine. Functional constipation and chronic functional abdominal pain are other disorders of the intestine that have physiological causes, but do not have identifiable structural, chemical, or infectious pathologies. They are aberrations of normal bowel function but not diseases.

• Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. Diverticular disease occurs when pouches form on the intestinal wall. Once the pouches become inflamed, it is known as diverticulitis, or diverticular disease.

• Endometriosis can affect the intestines, with similar symptoms to irritable bowel syndrome.

• Bowel twist (or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel infarction and death.

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.
• Moffatt, J. D. 2001. Variations on a theme: Specializations of the vertebrate digestive system. Hillfied Strathallan College. Retrieved December 17, 2007.
• Palaeos. 2003. Insectivora. Palaeos. Retrieved July 1, 2007.
• Ritchison, G. 2007. BIO 342, Comparative Vertebrate Anatomy: Lecture notes 7—Digestive system. Gary Ritchison's Home Page, Eastern Kentucky University. Retrieved November 23, 2007.
• Solomon, E. P., L. R. Berg, and D. W. Martin. 2002. Biology. Pacific Grove, CA: Brooks/Cole Thomson Learning. ISBN 0030335035
• Thomson, A., L. Drozdowski, C. Iodache, B. Thomson, S. Vermeire, M. Clandinin, and G. Wild. 2003. Small bowel review: Normal physiology, part 1. Dig Dis Sci 48(8): 1546-1564. Retrieved November 23, 2007.
• Thomson, A., L. Drozdowski, C. Iodache, B. Thomson, S. Vermeire, M. Clandinin, and G. Wild. 2003. Small bowel review: Normal physiology, part 2. Dig Dis Sci 48(8): 1565-1581. Retrieved November 23, 2007.
• Townsend, C. M., and D. C. Sabiston. 2004. Sabiston Textbook of Surgery: The Biological Basis of Modern Surgical Practice. Philadelphia: Saunders. ISBN 0721604099
• 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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