Editing Small intestine (section)

==Function==
Food from the stomach is allowed into the duodenum through the [[pylorus]] by a muscle called the [[pyloric sphincter]].

===Digestion===
The small intestine is where most chemical digestion takes place. Many of the [[digestive enzyme]]s that act in the small intestine are secreted by the [[pancreas]] and [[liver]] and enter the small intestine via the [[pancreatic duct]]. Pancreatic enzymes and [[bile]] from the gallbladder enter the small intestine in response to the hormone [[cholecystokinin]], which is produced in the response to the presence of nutrients. [[Secretin]], another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of [[bicarbonate]] into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.

The three major classes of nutrients that undergo digestion are [[protein]]s, [[lipid]]s (fats) and [[carbohydrate]]s:
*  Proteins are degraded into small [[peptide]]s and [[amino acid]]s before absorption.<ref>{{cite journal |author=Silk DB |title=Progress report. Peptide absorption in man|journal=Gut |volume=15|issue=6|pages=494–501|year=1974|pmid= 4604970 |pmc=1413009 |doi=10.1136/gut.15.6.494}}</ref> Chemical breakdown begins in the stomach and continues in the small intestine. Proteolytic enzymes, including [[trypsin]] and [[chymotrypsin]], are secreted by the [[pancreas]] and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic [[brush border]] enzyme, splits one amino acid at a time. [[Aminopeptidase]] and [[dipeptidase]] free the end amino acid products.
* Lipids (fats) are degraded into [[fatty acid]]s and [[glycerol]]. Pancreatic lipase breaks down [[triglyceride]]s into free fatty acids and [[monoglyceride]]s. Pancreatic lipase works with the help of the salts from the [[bile]] secreted by the [[liver]] and stored in the [[gall bladder]]. Bile salts attach to triglycerides to help [[emulsion|emulsify]] them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
* Some carbohydrates are degraded into simple sugars, or [[monosaccharide]]s (e.g., [[glucose]]). Pancreatic amylase breaks down some carbohydrates (notably [[starch]]) into oligosaccharides. Other carbohydrates pass undigested into the large intestine for further handling by [[intestinal bacteria]]. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose (a disaccharide), as well as most polysaccharides, is not digested in the small intestine. Some carbohydrates, such as [[cellulose]], are not digested at all, despite being made of multiple [[glucose]] units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large intestine.

===Absorption===
Digested food is now able to pass into the blood vessels in the wall of the intestine through either [[diffusion]] or [[active transport]]. 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 [[intestinal epithelium]], a [[simple columnar epithelium]]. Structurally, the mucosa is covered in wrinkles or flaps called [[circular folds]], which are considered permanent features in the mucosa. They are distinct from [[rugae]] which are considered non-permanent or temporary allowing for distention and contraction. From the circular folds project microscopic finger-like pieces of tissue called [[intestinal villus|villi]] ([[Latin]] for "shaggy hair"). The individual epithelial cells also have finger-like projections known as [[microvilli]]. The functions of the circular folds, the villi, and the microvilli are to increase the amount of surface area available for the absorption of [[nutrient]]s, and to limit the loss of said nutrients to intestinal fauna.

Each villus has a network of [[capillary|capillaries]] and fine lymphatic vessels called [[lacteal]]s 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 material that remains undigested and unabsorbed passes into the large intestine.

[[File:Absorption of glucose in the small intestine.svg|thumb|Absorption of glucose in the small intestine]]

Absorption of the majority of nutrients takes place in the [[jejunum]], with the following notable exceptions:
* [[Iron]] is absorbed in the duodenum.
* [[Folate]] (Vitamin B9) is absorbed in the duodenum and jejunum.
* [[Vitamin B12]] and [[bile salts]] are absorbed in the [[terminal ileum]]. Vitamin B12 will only be absorbed by the ileum after binding to a protein known as [[intrinsic factor]].
* Water is absorbed by [[osmosis]] and [[lipid]]s by passive diffusion throughout the small intestine.
* [[Sodium bicarbonate]] is absorbed by active transport and [[glucose]] and [[amino acid]] [[co-transport]]
* [[Fructose]] is absorbed by [[facilitated diffusion]].

===Immunological===
The small intestine supports the body's [[immune system]].<ref>{{cite web|url=http://www.massgeneral.org/about/pressrelease.aspx?id=1533|title=Intestinal immune cells play an unexpected role in immune surveillance of the bloodstream|publisher=Massachusetts General Hospital|date=13 December 2012|access-date=28 August 2013|archive-date=16 October 2018|archive-url=https://web.archive.org/web/20181016130005/https://www.massgeneral.org/about/pressrelease.aspx?id=1533|url-status=dead}}</ref>  The presence of [[gut flora]] appears to contribute positively to the host's immune system.
[[Peyer's patch]]es, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system.<ref>{{cite journal|last1=Canny|first1=G. O.|last2=McCormick|first2=B. A.|title=Bacteria in the Intestine, Helpful Residents or Enemies from Within?|journal=Infection and Immunity|volume=76|issue=8|year=2008|pages=3360–3373|issn=0019-9567|doi=10.1128/IAI.00187-08|pmc=2493210|pmid=18474643|citeseerx=10.1.1.596.7265}}</ref>