Artlabeling Activity the Wall of the Small Intestine 2 of 2

Introduction

The small intestine is a crucial component of the digestive system that allows for the breakdown and absorption of important nutrients that permits the body to function at its superlative functioning. The small intestine accomplishes this via a circuitous network of blood vessels, nerves, and muscles that piece of work together to accomplish this task. Information technology is a massive organ that has an boilerplate length of 3 to 5 meters. It divides into the duodenum, jejunum, and ileum.[1][2][iii]

• The duodenum is the shortest section, on boilerplate measuring from 20 cm to 25 cm in length. Its proximal end is continued to the antrum of the tummy, separated by the pylorus, and the distal terminate blends into the beginning of the jejunum. The duodenum surrounds the pancreas, in the shape of a "C" and receives chyme from the breadbasket, pancreatic enzymes, and bile from the liver; this is the only part of the pocket-size intestines where Brunner's glands are nowadays on histology.

• The jejunum is roughly two.5 meters in length, contains plicae circulares (muscular flaps), and villi to absorb the products of digestion.

• The ileum is the final portion of the minor intestine, measuring around 3 meters, and ends at the cecum. It absorbs whatsoever final nutrients, with major absorptive products beingness vitamin B12 and bile acids.

Layers of the Small Intestine

• Serosa: The serosa is the outside layer of the small intestine and consists of mesothelium and epithelium, which encircles the jejunum and ileum, and the anterior surface of the duodenum since the posterior side is retroperitoneal. The epithelial cells in the small intestine have a rapid renewal rate, with cells lasting for only three to 5 days.

• Muscularis: The muscularis consists of 2 smooth muscle layers, a sparse outer longitudinal layer that shortens and elongates the gut, and a thicker inner circular layer of smoothen muscle, which causes constriction. Nerves lie between these two layers and allow these to muscle layers to work together to propagate food in a proximal to distal direction.

• Submucosa: The submucosa consists of a layer of connective tissue that contains the blood vessels, nerves, and lymphatics.

• Mucosa: The mucosa is the innermost layer and is designed for maximal absorption by beingness covered with villi protruding into the lumen that increases the surface area. The crypt layer of the small bowel that is the area of continual prison cell renewal and proliferation. Cells move from the crypts to the villi and change into either enterocytes, goblet cells, Paneth cells, or enteroendocrine cells.

Of importance is the mesentery, which is a double fold of the peritoneum that not just anchors the pocket-sized intestines to the back of the abdominal wall, but also contains the blood vessels, nerves, and lymphatic vessels that supply the small intestine.[4][five]

Construction and Function

The principal office of the pocket-sized intestine is to intermission downward food, absorb nutrients needed for the body, and get rid of the unnecessary components. It also plays a role in the immune system, acting as a barrier to a multitude of flora that inhabits the gut and to make sure no harmful bacteria enter the body.

• The duodenum is the initial portion of the small intestine and is where absorption actually begins. It is ofttimes described as being split into four parts: superior, descending, horizontal, and ascending. The superior portion is the only section that is peritoneal; the rest is retroperitoneal. Pancreatic enzymes enter the descending duodenum via the hepatopancreatic ampulla and interruption downwardly chyme, a mix of stomach acid and food, from the stomach. Bicarbonate is also secreted into the duodenum to neutralize breadbasket acrid before reaching the jejunum. Lastly, the liver introduces bile into the duodenum, which allows for the breakdown and assimilation of lipids from food products. A significant landmark for the duodenum is the ligament of Trietz, a ligament made of skeletal musculus that tethers the duodenal-jejunal flexure to the posterior wall.

• The primary function of the jejunum is to absorb sugars, amino acids, and fatty acids. Both the jejunum and ileum are peritoneal.

• The ileum absorbs whatever remaining nutrients that did not get absorbed by the duodenum or jejunum, in item vitamin B12, besides as bile acids that will get on to be recycled.

Embryology

The small intestine comes from the archaic gut, which forms from the endodermal lining. The endodermal layer gives rise to the inner epithelial lining of the digestive tract, which is surrounded by the splanchnic mesoderm that makes up the muscular connective tissue and all the other layers of the small intestine. The jejunum and ileum come up from the midgut, whereas the duodenum derives from the foregut.

 Villi and crypts make upwardly the lining of the small-scale intestine. Originally, the pocket-size intestine is lined by cuboidal cells up until the ninth week of gestation; and so, villi brainstorm to form. Crypt formation begins between the 10th to twelfth weeks of gestation.

Blood Supply and Lymphatics

The arterial claret supply for the small-scale intestine showtime comes from the celiac trunk and the superior mesenteric artery (SMA).

• The superior pancreaticoduodenal artery is fed from the gastroduodenal artery, which branches from the proper hepatic avenue, which is traceable back to the celiac trunk. It anastomoses with the inferior pancreaticoduodenal avenue, which comes from the SMA, to supply blood to the duodenum.

• The jejunum and ileum receive their blood supply from a rich network of arteries that travel through the mesentery and originate from the SMA. The multitude of arterial branches that separate from the SMA is known as the arterial arcades, and they requite rise to the vasa recta that deliver the blood to the jejunum and ileum.

The venous blood mimics that of the arterial supply, which coalesces into the superior mesenteric vein (SMV), which so joins with the splenic vein to form the portal vein.

Lymphatic drainage starts at the mucosa of the small intestine, into nodes adjacent to the small-scale intestine in the mesentery, to nodes near the arterial arcades, then to nodes nearly the SMA/SMV. Lymph then flows into the cisterna chyli and then up the thoracic ducts, and then empties into the venous organization left internal jugular, and subclavian veins see. The lymphatic drainage of the small intestine is a major transport organization for absorbed lipids, the immune defense system, and the spread of cancer cells coming from the small intestine, explaining Virchow'south node enlargement from pocket-size intestine cancers.

Fretfulness

The nervous system of the small intestine is made upwards of the parasympathetic and sympathetic divisions of the autonomic nervous system. The parasympathetic fibers originate from the Vagus nerve and control secretions and motility. The sympathetic fibers come from three sets of splanchnic nerve ganglion cells located around the SMA. Motor impulses from these nerves command blood vessels, along with gut secretions and motility. Painful stimuli from the small intestine travel through the sympathetic fibers as well.

Muscles

Two layers of smooth muscle class the pocket-size intestine. The outermost layer is the thin, longitudinal musculus that contracts, relaxes, shortens, and lengthens the gut allowing nutrient to move in one direction. The innermost layer is a thicker, circular muscle. This layer enables the gut to contract and break autonomously larger food particles. It also stops nutrient from moving in the wrong direction by blocking the more than proximal finish. The two muscle layers work together to propagate food from the proximal end to the distal end.

Clinical Significance

• With shortening or destruction of the small intestine, there can be a decrease in the assimilation of essential vitamins, minerals, and other nutrients for the body that can cause a myriad of problems that can interfere with other systems of the body.[6][7][8]

• Duodenal ulcers tin be a serious health run a risk. They most commonly occur on the posterior wall of the superior portion of the duodenum. Complete ulceration of an ulcer through the wall of the duodenum can upshot in peritonitis and harm to the surrounding organs. The about common risk factor in developing these ulcers is infection withHelicobacter pylori.

• Wilke'due south syndrome, also known as SMA syndrome, occurs due to compression of the duodenum between the superior mesenteric artery and the abdominal aorta. This status creates an obstruction and oftentimes leads to nausea, vomiting, and abdominal hurting. Information technology nearly commonly presents in individuals who lack intra-abdominal fat, such as those suffering from anorexia nervosa or cancer.

• The ileocecal valve is an of import landmark for Meckel illness, an ileal diverticulum that, when nowadays, is ofttimes located shut to the valve. A common mnemonic device to retrieve the details of this pathology is by using the "Rules of 2's" (Ii years old, 2 anxiety from the ileocecal valve, two% of the population). It classically presents in a immature male patient with hematochezia and periumbilical pain. However, it can frequently be asymptomatic.

• Crohn disease is a chronic inflammatory disease that most often affects the ileum just tin can be institute throughout the GI tract (except for the rectum, which is generally spared).

Review Questions

Figure

Small intestine. Image courtesy S Bhimji MD

References

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Stallard DJ, Tu RK, Gould MJ, Pozniak MA, Pettersen JC. Minor vascular beefcake of the abdomen and pelvis: a CT atlas. Radiographics. 1994 May;14(iii):493-513. [PubMed: 8066265]

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Augustyn Thou, Grys I, Kukla K. Small intestinal bacterial overgrowth and nonalcoholic fatty liver affliction. Clin Exp Hepatol. 2019 Mar;five(i):1-x. [PMC free article: PMC6431096] [PubMed: 30915401]

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Gonsalves North. Eosinophilic Gastrointestinal Disorders. Clin Rev Allergy Immunol. 2019 Oct;57(2):272-285. [PubMed: 30903439]

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Barsouk A, Rawla P, Barsouk A, Thandra KC. Epidemiology of Cancers of the Small Intestine: Trends, Risk Factors, and Prevention. Med Sci (Basel). 2019 Mar 17;7(3) [PMC free article: PMC6473503] [PubMed: 30884915]

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Source: https://www.ncbi.nlm.nih.gov/books/NBK459366/