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Gastrointestinal Tract 4: Anatomy and Role of the Jejunum and Ileum

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Gastrointestinal Tract 4: Anatomy and Role of the Jejunum and Ileum Clinical Practice Keywords Villi/Microvilli/Absorption/ Segmentation/Vitamin B complex Systems of life This article has been GI tract double-blind peer reviewed In this article... ● Role of the jejunum and ileum in chemical digestion and absorption of nutrients ● Nutrient absorption from the small intestine to the bloodstream via the villi ● Processes of segmentation and peristalsis Gastrointestinal tract 4: anatomy and role of the jejunum and ileum Key points Authors Yamni Nigam is professor in biomedical science; John Knight is associate The small intestine professor in biomedical science; Nikki Williams is associate professor in respiratory comprises the physiology; all at the College of Human and Health Sciences, Swansea University. duodenum, jejunum and ileum Abstract After its passage through the duodenum, where most chemical digestion takes place, chyme passes through the jejunum and ileum. Their main role is to ensure The jejunum and that the various molecules resulting from chemical digestion pass through the gut ileum finish chemical wall into the blood or lymph. This process of nutrient absorption is helped by the digestion and presence of folds and projections that hugely increase the surface area of the gut absorb most of wall, and regular contractions of the rings of smooth muscle that move intestinal the nutrients contents back and forth. This article, the fourth in a six-part series exploring the gastrointestinal tract, describes the anatomy and functions of the jejunum and ileum. Folds and projections in the Citation Nigam Y et al (2019) Gastrointestinal tract 4: anatomy and role of the small intestine’s wall jejunum and ileum. Nursing Times; 115: 9, 41-44. increase the surface area for absorption ith the exception of inges- times the surface area of the skin – which Nutrients are tion, the small and large is essential for the absorption of nutrients transported across intestines carry out all the (Wilson, 2008). the gut wall into Wmajor functions of the The anatomy and function of the duo- the bloodstream digestive system. This is where the ‘real denum, the first part of the small intestine, passively or actively, business’ of digestion takes place. The is described in part 3 of this series on the GI sometimes with the intestines take up most of the space in the tract. Having received acidic chyme from help of carriers abdominal cavity and constitute the the stomach, the duodenum completes greatest portion of the gastrointestinal a large part of the process of chemical Peristalsis moves (GI) tract in terms of mass and length. They digestion, liberating small molecules from unabsorbed matter receive their blood supply through the ingested food (see part 3). Once this is towards the large mesenteric artery. done, the jejunum and ileum mainly intestine through the The small intestine is about five times assume the role of absorbing these mole- ileocaecal valve longer than the large intestine but has a cules (amino acids, monosaccharides and smaller diameter (about 2.54cm versus lipids), which pass into the bloodstream to 7.62cm), which is why it is called ‘small’. It be used by the body. This article, part 4 of comprises the duodenum (25cm), jejunum the series, describes the anatomy and (around 2.5m) and ileum (around 3.5m). functions of the jejunum and ileum. Tethered to the posterior wall of the abdomen by the mesentery (an extension Anatomy of the jejunum of the peritoneum), the entire convolution The jejunum makes up two-fifths of the of the small intestine lies loosely in the total length of the small intestine and is abdominal cavity, framed by the colon about 0.9m in length. It starts at the duo- (Fig 1). Its folds and the projections in its denojejunal flexure and ends at the ileum. lining create an enormous surface area of There is no clear border between the approximately 200m2 – more than 100 jejunum and the ileum. Histologically, the Nursing Times September 2019 / Vol 115 Issue 9 41 www.nursingtimes.net Clinical Practice Systems of life Fig 1. Anatomy of the small and large intestines The venules allow glucose and amino acids to be absorbed directly into the bloodstream, while products from the breakdown of lipids (fatty acids and glyc- erol) are absorbed into the lymphatic Stomach system via the lacteals. Small intestine Microvilli Large intestine The mucosal epithelial cells (Fig 3) have Duodenum thin, hair-like extensions about 1μm (0.001mm) in length, jutting out into the Jejunum Transverse intestinal lumen. These tiny projections colon are known as microvilli and there are Ileum Descending approximately 200 million of them per colon 1mm2. They expand the surface area avail- Ascending able for nutrient absorption by another 20 colon times. Microscopically, they appear as a Sigmoid mass of bristles and are, therefore, termed colon the brush border. Fixed to the surface of the microvilli are a series of enzymes that Rectum finish chemical digestion. Appendix Anatomy of the ileum Caecum The ileum is the longest part of the small intestine, making up about three- fifths of its total length. It is thicker and more vascular than the jejunum, and the circular folds are less dense and more sepa- jejunum differs from the rest of the small intestinal lumen (Fig 2), multiplying by 10 rated (Keuchel et al, 2013). At the distal intestine by the absence of Brunner’s the surface area available for nutrient end, the ileum is separated from the large glands (which are present in the duo- absorption. Each villus contains a: intestine by the ileocaecal valve, a denum – see part 3) and Peyer’s patches ● Capillary bed – comprising an arteriole sphincter formed by the circular muscle (which are present in the ileum – see part 1 and a venule; layers of the ileum and caecum, and con- and below). ● Lymphatic capillary – central lacteal trolled by nerves and hormones. The ileo- A vast surface area is a prerequisite for (Fig 3). caecal valve prevents reflux of the the optimal absorption of nutrients, so the wall of the jejunum contains the following Fig 2. Villi in intestinal mucosal lining features that increase its surface area: ● Circular folds; Villi ● Villi; ● Microvilli. These features are also found, albeit with slight differences, in the ileum. Circular folds Macroscopically noticeable are the numerous circular folds (or valves of Kerck- Mucosa ring) running parallel to each other in the mucosa of the jejunum. These deep ridges in the mucosal lining triple the surface area of the absorptive mucosa in the intestinal wall. They also slow down the flow of chyme, as their shape causes it to travel in a spiral fashion rather than moving down the GI tract in a straight line (Welcome, Submucosa 2018). This slowing down provides more time for nutrients to be absorbed. Muscularis Villi Located in the circular folds and meas- uring 0.5-1mm in length, finger-like pro- PETER LAMB jections known as villi extend into the Nursing Times September 2019 / Vol 115 Issue 9 42 www.nursingtimes.net Fig 3. Longitudinal view of a villus Microvilli increase surface area Short chain fatty acid for absorption Amino acid Monosaccharide Lacteal Chylomicron Blood capillary Venule Arteriole Blood Lymphatic vessel Lymph bacteria-rich content from the large intes- “The small and large concentration gradient – movement from tine into the small intestine. an area where they are in high concentra- The ileum is rich in immune tissue intestines carry out all the tion to one where they are in lower concen- (lymphoid follicles). A characteristic fea- major functions of the tration – in this case, the blood. Water and ture is Peyer’s patches, found lying in its digestive system” some vitamins can cross the gut wall pas- mucosa, which are an important part of sively. Active transport requires energy to gut-associated lymphoid tissue. One The rings of smooth muscle in the wall pull molecules out of the intestinal lumen Peyer’s patch is around 2-5cm long and of the small intestine repeatedly contract against a concentration gradient. In addi- consists of around 300 aggregated lym- and relax in a process called segmentation. tion, certain molecules – such as glucose, phoid follicles. These are concentrated in This moves intestinal contents back and amino acids and vitamin B12 – have their the distal ileum and serve to keep bacteria forth. Segmentation distends the small own carriers or transporters, which they from entering the bloodstream. intestine but does not drive chyme use to ‘piggyback’ across the gut wall into Peyer’s patches are most prominent in through the tract; instead, it mixes it with the bloodstream. young people and become less distinct digestive juices and then pushes it against with age, which reflects the age-related the mucosa to allow nutrient absorption. Carbohydrates reduction in activity of the gut’s immune Each day, approximately 8L of water Digested carbohydrates enter the blood system. (from dietary ingestion as well as GI tract capillaries irrigating each villus. Almost secretions and juices, including saliva), all ingested carbohydrates are absorbed as Digestion and absorption several hundred grams of carbohydrates, monosaccharides, 80% of which are glu- The duodenum accomplishes a good deal ≥100g of fat, 50-100g of amino acids and cose. Glucose is actively absorbed via a co- of chemical digestion, as well as a small 50-100g of salt ions pass through the wall transport mechanism using sodium ions amount of nutrient absorption (see part 3); of the small intestine and into the blood as carriers. Other absorbable monosaccha- the main function of the jejunum and (Hall, 2011). rides include galactose from milk and ileum is to finish chemical digestion (enzy- The transport of nutrients across the fructose from fruit. matic cleavage of nutrients) and absorb membranes of the intestinal epithelial these nutrients along with water and vita- cells into the villi, and subsequently into Amino acids mins.
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