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Home , Circular folds, Gastrointestinal tract, Ileum, Jejunum, Small intestine

Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use

Clinical Practice Keywords Villi/Microvilli/Absorption/ Segmentation/ B complex Systems of life This article has been GI tract double-blind peer reviewed In this article... ● Role of the and in chemical and absorption of ●  absorption from the to the bloodstream via the villi ● Processes of segmentation and

Gastrointestinal tract 4: 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 and Health Sciences, Swansea University. , jejunum and ileum Abstract After its passage through the duodenum, where most chemical digestion takes place, 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 ileum finish chemical wall into the blood or . 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 that move intestinal the nutrients contents back and forth. This article, the fourth in a six-part series exploring the , 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 [online]; 115: 9, 43-46. increase the surface area for absorption ith the exception of inges- times the surface area of the – 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 and constitute the the , 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 . 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 but has a cules (amino acids, monosaccharides and smaller diameter (about 2.54cm versus ), 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 by the (an extension Anatomy of the jejunum of the ), 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

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Fig 1. Anatomy of the small and large intestines The venules allow 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 . 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 . 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 that 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 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 (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 l  bed – comprising an arteriole 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). l Lymphatic capillary – central trolled by and . 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: l Circular folds; Villi l Villi; l 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, 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 PETER jections known as villi extend into the

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Fig 3. Longitudinal view of a villus

Microvilli increase surface area Short chain for absorption

Amino acid

Monosaccharide Lacteal Chylomicron

Blood capillary Venule

Arteriole Blood

Lymphatic vessel Lymph

-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 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 can cross the gut wall pas- mucosa, which are an important part of sively. 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 – 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. reduction in activity of the gut’s immune Each day, approximately 8L of water Digested carbohydrates enter the blood system. (from dietary as well as GI tract irrigating each villus. Almost and juices, including ), 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 and ileum is to finish chemical digestion (enzy- The transport of nutrients across the 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. The brush border of the small intes- blood capillaries and lacteals, occurs either Most products of digestion (amino tine contains enzymes that complete the passively or actively. Passive transport acids) are also absorbed through an active process of chemical digestion. Table 1 lists requires no energy and involves the diffu- co-transport mechanism with sodium

PETER LAMB PETER these enzymes and their roles. sion of simple molecules along a ions and enter the blood capillary system

Nursing Times [online] September 2019 / Vol 115 Issue 9 45 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice For more articles on , go to Systems of life nursingtimes.net/gastroenterology of each villus. They then travel to the Table 1. Brush border enzymes of the small intestine and their via the hepatic portal . role in chemical digestion Fats Activity Digested fats mingle with salts, which Maltase Digests the disaccharide maltose into two molecules of glucose ferry them to the mucosa where they are coated with and aggregated Digests the disaccharide sucrose into glucose and fructose into small molecules called chylomicrons, Digests the disaccharide into glucose and galactose which are taken into the central lacteals of Intestinal Digests fats into fatty acids and the villi. They travel with lymph to the tho- racic duct, where they enter the blood Intestinal Completes protein digestion by digesting into their supply. If there is of fats, peptidases components these pass into the large intestine, where they form pale, oily, foul-smelling stools Vitamin B12 is liberated from ingested the ileum increases, the ileocaecal valve (steatorrhoea – see part 3). When that hap- food in the acid milieu of the stomach. In relaxes, allowing food residue to enter the pens, certain fat-soluble vitamins (A, D, E the duodenum, it binds with intrinsic large intestine at the caecum. NT and K) may also not be absorbed, poten- factor produced by the gastric parietal tially leading to deficiencies. cells (see part 2); it is only in that bound ● Part 5 of our six-part series on the form that it can be absorbed (Moll and anatomy and physiology of the 200m2 Davis, 2017). Absorption occurs in the ter- gastrointestinal tract will describe the QUICK Approximate surface area minal portion of the ileum, where vitamin anatomy and functions of the large FACT of the intestines available B12 attaches to specific membrane recep- intestine, as well as common for nutrient absorption tors located on absorptive cells (entero- of the small and large intestines. cytes) at the bottom of the pits between the microvilli (Schjønsby, 1989). To leave the References Hall JE (2011) Digestion and absorption in the Vitamin B complex and enter the bloodstream, the gastrointestinal tract. In: Guyton and Hall Textbook The vitamin B complex encompasses eight vitamin must then bind to a carrier pro- of Medical Physiology. Philadelphia, PA: Saunders. Keuchel M et al (2013) Normal small bowel. Video water-soluble vitamins that are essential tein, transcobalamin II. Journal and Encyclopedia of GI ; 1: 1, for key functions of the body, including A common cause of vitamin B12 defi- 261-263. red blood cell formation, maintenance of ciency is the destruction of gastric parietal Moll R, Davis B (2017) , vitamin B12 and . ; 45: 4, 198-203. healthy hair and nails, and healthy func- cells by autoantibodies, which severely Schjønsby H (1989) Vitamin B12 absorption and tioning of the brain and . These eight reduces production by the malabsorption. Gut; 30: 12, 1686-1691. vitamins are: B1 (thiamine), B2 (riboflavin), stomach and leads to a condition known as Welcome MO (2018) Structural and functional organization of the gastrointestinal tract. In: B3 (), B5 (pantothenic acid), B6 (pyri- pernicious anaemia (see part 2). Vitamin Gastrointestinal Physiology: Development, doxine), B7 (), B9 (folate) and B12 B12 deficiency should not be ignored. If Principles and Mechanisms of Regulation. Cham: (cobalamin). individuals who are deficient do not Springer International Publishing. Wiley KD, Gupta M (2019) Vitamin B1 Thiamine receive injections of the vitamin they may Deficiency (Beriberi). Treasure Island, FL: Vitamin B1. Essential for , experience severe negative consequences, StatPearls. Bit.ly/StatPearlsThiamine Wilson M (2008) The indigenous microbiota of vitamin B1 also plays a role in healthy including dementia. the gastrointestinal tract. In: Bacteriology of conduction and . : An Ecological Perspective. Oxford: It is found in fortified foods such as bread Movement towards the large Wiley-Blackwell. Young KA et al (2014) The small and large and cereals, but also in eggs, , nuts, intestine intestines. In: Anatomy and Physiology. Houston, legumes and certain meats (Wiley and Digestive activity in the stomach provokes TX: OpenStax College. Gupta, 2019). Vitamin B1 deficiency is the gastroileal reflex, which stimulates common in people who have a poor diet peristalsis to push contents along the (for example, homeless people) and can ileum and the colon. The reflex ensures For more on this topic online cause a range of disorders including beri- that the content of one meal is completely l Anatomy and physiology of ageing 3: beri. In some cases, vitamin B1 deficiency emptied from both the stomach and the the digestive system can be caused by long-term, heavy small intestine before the next meal is Bit.ly/NTDigestiveSOL intake, which eventually impairs the eaten. It can take up to five hours for all body’s ability to absorb the vitamin. chyme to leave the small intestine (Young

Vitamin B1 deficiency caused by alcohol et al, 2014). CLINICAL can result in Wernicke’s encephalopathy When most of the chyme has been SERIES Gastrointestinal tract series or Korsakoff’s psychosis. absorbed, the walls of the small intestine Part 1: The and oesophagus Jun become less distended and segmentation Bit.ly/NTGITract1 Vitamin B12. This vitamin is essential for gives way to peristalsis, which helps move Part 2: The stomach Jul red blood cell development, normal func- unabsorbed matter along towards the Bit.ly/NTGITract2 tioning of the , cell metab- large intestine. Peristalsis works a little Part 3: The duodenum, liver and Aug olism and DNA synthesis. The richest nat- like squeezing along and out of Bit.ly/NTGITract3 ural sources of vitamin B12 are liver and a tube. With each repeated peristaltic con- Part 4: The jejunum and ileum Sep , but it is also present in meat, fish, traction, chyme and waste slowly move Part 5: The large intestine Oct Part 6: Gut microbes Nov dairy products, eggs and shellfish. down the small intestine. When in

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