Small intestine – Wikipedia

Organ in the gastrointestinal tract
The small intestine or small bowel is an organ in the gastrointestinal tract where most of the preoccupation of nutrients from food takes identify. It lies between the stomach and big intestine, and receives bile and pancreatic juice through the pancreatic duct to help in digestion. The small intestine is about 5.5 metres ( 18 feet ) long and close up many times to fit in the abdomen. Although it is longer than the big 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 villus begins. [ 2 ] The jejunum is specialized for the assimilation through its lining by enterocytes : minor nutrient particles which have been previously digested by enzymes in the duodenum. The independent function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion that were not absorbed by the jejunum .

social organization [edit ]

size [edit ]

The length of the belittled intestine can vary greatly, from deoxyadenosine monophosphate short as 3 metres ( 10 feet ) to adenine farseeing as 10.5 thousand ( 34+1⁄2 foot ), besides depending on the measurement proficiency used. [ 3 ] The distinctive length in a living person is 3–5 molarity ( 10–16+1⁄2 foot ). [ 4 ] [ 5 ] The duration depends both on how tall the person is and how the length is measured. [ 3 ] Taller people by and large have a longer small intestine and measurements are broadly longer after death and when the intestine is empty. [ 3 ]

Small bowel dilation on CT scan in adults[6]
<2.5 cm Non-dilated
2.5-2.9 cm Mildly dilated
3–4 cm Moderately dilated
>4 cm Severely dilated

It is approximately 1.5 centimetres ( 5⁄8 column inch ) in diameter in newborns after 35 weeks of gestational senesce, [ 7 ] and 2.5–3 centimeter ( 1–1+1⁄8 in ) in diameter in adults. On abdominal X-rays, the little intestine is considered to be abnormally dilated when the diameter exceeds 3 curium. [ 8 ] [ 9 ] On CT scans, a diameter of over 2.5 curium is considered abnormally dilated. [ 8 ] [ 10 ] The surface area of the human small intestinal mucous membrane, due to enlargement caused by folds, villus and microvilli, averages 30 squarely metres ( 320 sq foot ). [ 11 ]

Parts [edit ]

The modest intestine is divided into three structural parts .
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. [ 13 ]

Blood supply [edit ]

The belittled intestine receives a blood issue from the coeliac torso and the victor mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the coeliac trunk via the lake superior pancreaticoduodenal artery and from the lake superior mesenteric artery via the subscript pancreaticoduodenal artery. These two arteries both have anterior and buttocks branches that meet in the midplane and anastomose. The jejunum and ileum receive blood from the superscript mesenteric artery. [ 14 ] Branches of the superior mesenteric artery form a series of arches within the mesentery known as arterial arcades, which may be respective layers deep. Straight blood vessels known as vessel recta travel from the arcades closest to the ileum and jejunum to the organs themselves. [ 14 ]

Microanatomy [edit ]

The three sections of the small intestine front like to each other at a microscopic charge, but there are some crucial differences. The parts of the intestine are as follows :
This cross part diagram shows the 4 layers of the small intestine wall .

Gene and protein expression [edit ]

About 20,000 protein coding genes are expressed in human cells and 70 % of these genes are expressed in the normal duodenum. [ 15 ] [ 16 ] Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the modest intestine. The represent specific proteins are expressed in glandular cells of the mucous membrane, such as fatty acid binding protein FABP6. Most of the more specifically expressed genes in the little intestine are besides expressed in the duodenum, for example FABP2 and the DEFA6 protein expressed in secretory granules of Paneth cells. [ 17 ]

Development [edit ]

The small intestine develops from the midgut of the archaic catgut metro. [ 18 ] By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a u-shaped fold called the primary intestinal loop. The loop grows therefore debauched in length that it outgrows the abdomen and protrudes through the navel. By workweek 10, the loop retracts binding into the abdomen. between weeks six and ten the humble intestine rotates counterclockwise, as viewed from the presence of the embryo. It rotates a far 180 degrees after it has moved back into the abdomen. This serve creates the twist determine of the big intestine. [ 18 ]

function [edit ]

food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter .

digestion [edit ]

The little intestine is where most chemical digestion takes rate. many of the digestive enzymes that act in the belittled 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 modest intestine in answer to the hormone cholecystokinin, which is produced in the reaction to the presence of nutrients. Secretin, another hormone produced in the small intestine, causes extra effects on the pancreas, where it promotes the spill of bicarbonate into the duodenum in ordering 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 :

  • Proteins are degraded into small peptides and amino acids before absorption.[19] 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 acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. 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 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 monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine and 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.

assimilation [edit ]

Digested food is now able to pass into the blood vessels in the wall of the intestine through either dissemination or active enchant. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucous membrane, of the small intestine, is lined with simple columnar epithelial tissue. structurally, the mucous membrane is covered in wrinkles or flaps called round folds, which are considered permanent features in the mucous membrane. They are distinct from ruga which are considered non-permanent or temp allowing for distention and contraction. From the circular folds project microscopic finger-like pieces of tissue called villus ( Latin for “ bushy hair ” ). The individual epithelial cells besides have finger-like projections known as microvilli. The functions of the round folds, the villus, and the microvilli are to increase the measure of come on area available for the absorption of nutrients, and to limit the loss of said nutrients to intestinal animal. Each villus has a network of capillaries and fine lymphatic vessels called lacteals near to its surface. The epithelial cells of the villus tape drive nutrients from the lumen of the intestine into these capillaries ( amino acids and carbohydrates ) and lacteals ( lipids ). The absorb 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.

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absorption of glucose in the little intestine absorption of the majority of nutrients takes plaza in the jejunum, with the following luminary exceptions :

immunological [edit ]

The little intestine supports the body ‘s immune system. [ 20 ] The presence of intestine plant appears to contribute positively to the host ‘s immune organization. Peyer ‘s patches, located within the ileum of the humble intestine, are an crucial region of the digestive tract ‘s local immune organization. They are region of the lymphatic system, and provide a locate for antigens from potentially harmful bacteria or other microorganisms in the digestive nerve pathway to be sampled, and subsequently presented to the immune organization. [ 21 ]

clinical significance [edit ]

The belittled intestine is a complex organ, and as such, there are a identical big number of possible conditions that may affect the routine of the little intestine. A few of them are listed below, some of which are common, with up to 10 % of people being affected at some meter in their lives, while others are vanishingly rare .

other animals [edit ]

The little intestine is found in all tetrapods and besides in teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a one-half times the duration of the pisces ‘s consistency. It normally has a issue of pyloric caeca, belittled pouch-like structures along its duration that assistant to increase the overall coat area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the minor intestine and the rectum being marked entirely by the end of the digestive epithelium. [ 22 ] In tetrapods, the ileocaecal valve is always salute, opening into the colon. The duration of the small intestine is typically longer in tetrapods than in teleosts, but is specially sol in herbivores, adenine well as in mammals and birds, which have a higher metabolic pace than amphibians or reptiles. The line of the modest intestine includes microscopic folds to increase its coat sphere in all vertebrates, but merely in mammals do these develop into true villus. [ 22 ] The boundaries between the duodenum, jejunum, and ileum are slightly obscure evening in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are basically arbitrary. [ 22 ] There is no minor intestine as such in non-teleost pisces, such as sharks, sturgeons, and lungfish. rather, the digestive part of the gut forms a spiral intestine, connecting the digest to the rectum. In this type of intestine, the intestine itself is relatively straight but has a farseeing fold running along the inner surface in a spiral manner, sometimes for dozens of turns. This valve greatly increases both the surface sphere and the effective duration of the intestine. The line of the helix intestine is like to that of the humble intestine in teleosts and non-mammalian tetrapods. [ 22 ] In lampreys, the spiral valve is highly little, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for about the integral length of the intestine, which is not subdivided into different regions. [ 22 ]

society and culture [edit ]

In traditional taiwanese medicine, the small intestine is a yang organ. [ 23 ]

extra images [edit ]

References [edit ]

bibliography [edit ]

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