Small intestine

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Small Intestine
Stomach colon rectum diagram.svg
Diagram showing the small intestine
LatinIntestinum tenue
Gray'ssubject #248 1168
ArterySuperior mesenteric artery
VeinHepatic portal vein
Nerveceliac ganglia, vagus [1]
LymphIntestinal lymph trunk
MeSHSmall+intestine
Dorlands/ElsevierSmall intestine
 
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Small Intestine
Stomach colon rectum diagram.svg
Diagram showing the small intestine
LatinIntestinum tenue
Gray'ssubject #248 1168
ArterySuperior mesenteric artery
VeinHepatic portal vein
Nerveceliac ganglia, vagus [1]
LymphIntestinal lymph trunk
MeSHSmall+intestine
Dorlands/ElsevierSmall intestine

The small intestine (or small bowel) is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to describe the entire intestine. This article is primarily about the human gut, though the information about its processes is directly applicable to most placental mammals. The primary function of the small intestine is the absorption of nutrients and minerals found in food. [2] (A major exception to this is cows; for information about digestion in cows and other similar mammals, see ruminants.)

Small intestine

Contents

Size and divisions

The average length of the small intestine in an adult human male is 6.9 m (22 feet 6 inches), and in the adult female 7.1 m (23 feet 4 inches). It can vary greatly, from as short as 4.6 m (15 feet) to as long as 9.8 m (32 feet).[3][4] It is approximately 2.5–3 cm in diameter.

The small intestine is divided into three structural parts:

Histology

Micrograph of the small intestine mucosa showing the intestinal villi and crypts of Lieberkühn.

The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The parts of the intestine are as follows:

LayerDuodenumJejunumIleum
serosa1st part serosa, 2nd - 4th adventitianormalnormal
muscularis externalongitudinal and circular layers, with Auerbach's (myenteric) plexus in betweensame as duodenumsame as duodenum
submucosaBrunner's glands and Meissner's (submucosal) plexusno BGno BG
mucosa: muscularis mucosaenormalnormalnormal
mucosa: lamina propriano PPno PPPeyer's patches
mucosa: intestinal epitheliumsimple columnar. Contains goblet cells, Paneth cellsSimilar to duodenum. Villi very long.Similar to duodenum. Villi very short.

Digestion and absorption

Food from the stomach is allowed into the duodenum by a muscle called the pylorus, or pyloricistalsis.

Digestion

The small intestine is where most chemical digestion takes place. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct. Enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid coming from the stomach.

The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates:

Absorption

Digested food is now able to pass into the blood vessels in the wall of the intestine through the process of diffusion. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or folds called plicae circulares, which are considered permanent features in the wall of the organ. They are distinct from rugae which are considered non-permanent or temporary allowing for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as microvilli. The function of the plicae circulares, the villi and the microvilli is to increase the amount of surface area available for the absorption of nutrients.

Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The food that remains undigested and unabsorbed passes into the large intestine.

Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:

Conditions affecting the small intestine

The small intestine is a complex organ, and as such, there are a very large number of possible conditions that may affect the function of the small bowel. A few of them are listed below, some of which are common, with up to 10% of people being affected at some time in their lives, while others are vanishingly rare.

In other animals

The small intestine is found in all tetrapods and also in teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium.[6]

In tetrapods, the ileocaecal valve is always present, opening into the colon. The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in herbivores, as well as in mammals and birds, which have a higher metabolic rate than amphibians or reptiles. The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi.[6]

The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary.[6]

There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.[6]

In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.[6]

See Also

References

Notes

  1. ^ Template:GeorgiaPjulianapoophysiologjy
  2. ^ http://www.britannica.com/EBchecked/topic/275485/human-body
  3. ^ "Elsevier: Gray's Anatomy, 40th Edition". http://www.bartleby.com/107/248.html#txt168.
  4. ^ "Lea Brothers and Co. 1907: Surgical Applied Anatomy". http://www.archive.org/stream/surgicalapplieda1907trev#page/n7/mode/2up.
  5. ^ Silk DB (1974). "Progress report. Peptide absorption in man". Gut 15 (6): 494-501. PMC 1413009. PMID 4604970. //www.ncbi.nlm.nih.gov/pmc/articles/PMC1413009/.}
  6. ^ a b c d e Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 349–353. ISBN 0-03-910284-X.

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