Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Keywords Liver/Bile/Pancreas/ Alkaline/Enzymatic digestion Systems of life This article has been GI tract double-blind peer reviewed In this article... l Role of hormones and enzymes within the duodenum l Role of liver and pancreas as accessory organs of the digestive system l Common conditions affecting the duodenum, liver and pancreas Gastrointestinal tract 3: the duodenum, liver and pancreas Key points Authors John Knight is associate professor in biomedical science; Nikki Williams is The duodenum is associate professor in respiratory physiology; Yamni Nigam is professor in biomedical the first segment of science; all at the College of Human Health and Sciences, Swansea University. the small intestine; the liver and Abstract After its passage through the stomach, ingested food turned into acidic pancreas are the chyme arrives in the first segment of the small intestine, a U-shaped tube called the two major accessory duodenum. The duodenum produces hormones and receives secretions from the organs of the gut liver (bile) and pancreas (pancreatic juice containing digestive enzymes). These various hormones, fluids and enzymes facilitate chemical digestion in the duodenum The major role of while also ensuring the acidity of chyme coming from the stomach is neutralised. This the duodenum is as is crucial to avoid autodigestion and corrosive damage to the lining of the gut. This a vessel for chemical article, the third in a six-part series exploring the gastrointestinal tract, describes the digestion and, to a anatomy, functions and common pathologies of the duodenum, liver and pancreas. lesser extent, the absorption of Citation Knight J et al (2019) Gastrointestinal tract 3: the duodenum, liver and minerals pancreas. Nursing Times [online]; 115: 8, 56-60. Chemical digestion in the duodenum art 1 and part 2 in our series about from the superior duodenal flexure; relies on bile the gastrointestinal (GI) tract approximately halfway down is the (secreted by the looked at the role of the mouth, major duodenal papilla, the entry point liver) and pancreatic Poesophagus and stomach in of the common bile duct and pancreatic juice (secreted by mechanical and chemical digestion. This duct, which fuse before entering the the pancreas) third part of the series covers the anatomy duodenum. The entry of bile and and function of the duodenum – the first pancreatic juice into the duodenum is To prevent segment of the small intestine – and the regulated by a small ring of muscle, the autodigestion and role of the two major accessory organs of sphincter of Oddi; corrosion, gastric digestion, the liver and the pancreas. It l Horizontal or transverse region: the acidity is neutralised also describes common pathologies that largest section of the duodenum in the duodenum by affect these parts of the GI tract. (10-12cm long) and the main area of pancreatic juice mineral absorption; Anatomy of the duodenum l Ascending region: this passes slightly Patients with The duodenum (Fig 1) is U-shaped and upwards into the abdominal cavity impaired digestion approximately 25-38cm long (Lopez and before connecting to the jejunum at the of fats may lack Khorasani-Zadeh, 2019). It consists of four duodenojejunal flexure. vitamins A, D, E regions: The duodenum is primarily a region of and K l Superior region: an enlarged upper area chemical digestion. It receives secretions of around 2cm called the duodenal from the liver and pancreas, and its bulb, which continues the pyloric mucosa contains large numbers of mucus- sphincter and is connected to the liver producing (goblet) cells and Brunner’s by the hepatoduodenal ligament; glands, which secrete a watery fluid that is l Descending region: this extends rich in mucus and bicarbonate ions. As in downwards into the abdominal cavity the stomach (see part 2), mucus acts as a Nursing Times [online] August 2019 / Vol 115 Issue 8 56 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Physiology of the duodenum is a polypeptide comprising around 42 amino acids. As its name suggests, GIP Superior region Pyloric sphincter inhibits the release of gastric secretions in the stomach, helping to avoid damage to Duodenal bulb the small intestine. It also promotes the Stomach release of insulin by the beta cells of the Ascending region pancreas (Pederson and McIntosh, 2016). Circular fold Duodenojejunal flexure Vasoactive intestinal peptide Descending VIP is a neuropeptide consisting of 28 region amino acids and produced by nerve cells Jejunum throughout the GI tract. Its role is poorly Major understood but it is thought to regulate duodenal papilla the composition of pancreatic juice and bile, as well as blood flow in gut tissue. Sphincter of Oddi However, VIP also has effects outside of the Horizontal region GI tract: it induces peripheral vasodilation, thereby lowering blood pressure, while increasing the strength of myocardial con- protective barrier against autodigestion ranging from four to 83. It is released by traction. It is also thought to play a role in and lubricates the passage of chyme. the enteroendocrine I-cells of the duo- regulating immune responses (Igarashi et The mucosal lining of the duodenum denum and jejunum. As Rehfeld (2017) al, 2011). also contains tall columnar epithelial cells explains, CCK: and extends into circular folds and finger- l Inhibits gastric emptying; Chemical digestion in the like projections (villi), which increase the l Helps prevent the duodenum overfilling; duodenum surface area for nutrient absorption. How- l Stimulates the production of Chemical digestion in the duodenum ever, nutrient absorption is not the duode- pancreatic enzymes; relies on the secretions of the two major num’s speciality and occurs mainly further l Stimulates the contraction of the accessory organs of the GI tract, the: down the small intestine in the jejunum smooth muscle wall of the gall bladder, l Liver, which produces bile; and ileum, where the villi are longer, more prompting it to release bile; l Pancreas, which produces pancreatic numerous and found at greater density. l Relaxes the sphincter of Oddi, allowing juice. bile and pancreatic juice to be released Bile and pancreatic juice are mixed and Hormone production in the into the duodenum; discharged into the duodenum every time duodenum l Enhances the activity of Brunner’s the sphincter of Oddi opens under the The pyloric sphincter, which separates the glands, increasing their output of influence of CCK. stomach and duodenum, periodically bicarbonate-rich secretions; opens to release small portions of acidic l Reduces sensations of hunger, together Liver and bile production chyme (see part 2). This sudden increase in with other hormones such as The liver is the largest internal organ of the acidity stimulates the release of several pancreatic polypeptide. human body; it weighs around 1.3kg in hormones including: women and 1.8kg in men. Located in the l Secretin; Gastric inhibitory polypeptide upper right quadrant of the abdomen, just l Cholecystokinin; Synthesised by the enteroendocrine below the diaphragm, it has over 500 docu- l Gastric inhibitory polypeptide (GIP); K-cells of the duodenum and jejunum, GIP mented functions. Its major role in l Vasoactive intestinal peptide (VIP). Fig 2. Stomach, liver, pancreas and duodenum Secretin Secretin, a peptide hormone comprising 27 amino acids, is secreted by the entero- Liver endocrine S-cells of the duodenum and Stomach jejunum. It stimulates the production and release of bicarbonate ions by the pan- creas; these accumulate in the pancreatic Gallbladder Pancreas juice, giving it an alkaline pH of around 8-8.6. Neutralising the acidity of chyme Common bile prevents damage to the mucosa in subse- duct quent sections of the GI tract and provides Sphincter of the Pancreatic duct a pH favourable to the activity of pancre- common bile duct atic enzymes (Jun et al, 2016). Descending region of the duodenum Cholecystokinin Cholecystokinin (CCK) exists in several Sphincter of Oddi Sphincter of the pancreatic duct PETER LAMB forms with numbers of amino acids Nursing Times [online] August 2019 / Vol 115 Issue 8 57 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Systems of life digestion is the production of bile, which Fig 3. Emulsification of fat by bile salts is concentrated and stored in the gall bladder (Fig 2). In a healthy person, the liver releases around 600ml of bile into the duodenum each day. This is done in small amounts as the gall bladder contracts in response to CCK (Hundt et al, 2019). Large fat globule Bile is a yellowish green fluid that con- tains mostly: l Water; l Bile salts, including sodium taurocholate and sodium glycocholate; l Cholesterol; l Phospholipids; l Bile pigments bilirubin and biliverdin, Emulsion of tiny which come from the breakdown of Bile salts fat droplets erythrocytes. Bile salts play an important role in chemical digestion in the duodenum. They lower the surface tension of large fat glob- l Bicarbonate ions; breaking down starch into maltose. As it ules, causing them to collapse into smaller l Pancreatic enzymes. catalyses starch digestion in the duo- droplets (Fig 3). This process is called One of its key roles is to neutralise denum lumen (inside space of the duo- emulsification, as it creates an emulsion of acidity in the chyme coming through from denum), it is also called luminal amylase tiny fat droplets with a larger surface area the stomach. This is essential to prevent (Williams, 2019). for subsequent breakdown by fat- autodigestion and ulceration of subse- Pancreatic amylase can function over a digesting enzymes (lipases). The digestion quent sections of the GI tract.