The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter.
The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called intestinal villi begins.[ human body | Britannica.com] The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by enzymes in the duodenum. The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion that were not absorbed by the jejunum.
Structure
Size
The length of the small intestine can vary greatly, from as short as to as long as , also depending on the measuring technique used.
[ The typical length in a living person is .][ Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty.]+ Small bowel dilation on CT scan in adults |
| Non-dilated |
| Mildly dilated |
| Moderately dilated |
| Severely dilated |
It is approximately in diameter in neonate after 35 weeks of gestational age,
Parts
The small intestine is divided into three structural parts.
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The duodenum is a short structure ranging from in length, and shaped like a "C".
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The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about long, and contains the circular folds, and intestinal villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum.
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The ileum: The final section of the small intestine. It is about 3 m (9.8 feet) long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the ileocecal junction.
The jejunum and ileum are suspended in the abdominal cavity by mesentery. The mesentery is part of the peritoneum. Arteries, veins, lymph vessels and nerves travel within the mesentery.
Blood supply
The small intestine receives a blood supply from the celiac trunk and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the celiac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and Anastomosis. The jejunum and ileum receive blood from the superior mesenteric artery.
Microanatomy
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:
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| Normal |
| Same as duodenum |
| No BG |
| Normal |
| Peyer's patches |
| Similar to duodenum, but the intestinal villus is short |
Gene and protein expression
About 20,000 protein coding genes are Gene expression in human cells and 70% of these genes are expressed in the normal duodenum.
Development
The small intestine develops from the midgut of the primitive gut tube.
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 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 , (fats) and :
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Proteins are degraded into small and before absorption.
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Lipids (fats) are degraded into and glycerol. Pancreatic lipase breaks down into free fatty acids and . 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 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.
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Some carbohydrates are degraded into simple sugars, or (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 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 , and to limit the loss of said nutrients to intestinal fauna.
Each villus has a network of capillary and fine lymphatic vessels called 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.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
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Iron is absorbed in the duodenum.
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Folate (Vitamin B9) is absorbed in the duodenum and jejunum.
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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.
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Water is absorbed by osmosis and by passive diffusion throughout the small intestine.
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Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport
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Fructose is absorbed by facilitated diffusion.
Immunological
The small intestine supports the body's immune system.
Clinical significance
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.
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Small intestine obstruction or obstructive disorders
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Infectious diseases
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(cancers)
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Developmental, congenital or genetic conditions
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Other conditions
Other animals
The small intestine is found in all and also in , 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.[
]
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 , as well as in mammals and , which have a higher metabolic rate than or . 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.[
]
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.
There is no small intestine as such in non-teleost fish, such as , , 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.[
]
In , 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.[
]
Society and culture
In traditional Chinese medicine, the small intestine is a yang organ.
Additional images
, greater omentum folded upwards.]]
Bibliography
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Solomon et al. (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning
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Townsend et al. (2004) Sabiston Textbook of Surgery, Elsevier
External links
