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You are watching: The major role of absorption in the ileum is to reclaim bile salts to be recycled back to the liver

StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

Bile is a physiological aqueous solution produced and secreted by the liver. It consists mainly the bile salts, phospholipids, cholesterol, conjugated bilirubin, electrolytes, and also water <1>. Bile travels through the liver in a collection of ducts, ultimately exiting through the common hepatic duct. Bile flows v this duct into the gallbladder, wherein it is concentrated and also stored. When engendered by the hormone cholecystokinin (CCK), the gallbladder contracts, pushing bile with the cystic duct and also into the typical bile duct.  Simultaneously, the sphincter the Oddi relaxes, permit bile to get in the duodenal lumen. The hormone secretin likewise plays an important role in the flow of bile right into the small intestine. By stimulating biliary and pancreatic ductular cell to secrete bicarbonate and water in response to the visibility of mountain in the duodenum, secretin effectively expands the volume the bile beginning the duodenum. In the tiny intestine, bile acids facilitate lipid digestion and also absorption. Only approximately 5% of these bile mountain are eventually excreted. The bulk of bile mountain are successfully reabsorbed indigenous the ileum, secreted into the portal venous system, and also returned come the liver in a process known together enterohepatic recirculation <2><3><4>.

Formation

Bile

Bile is produced by hepatocytes and also it is then modified by the cholangiocytes lining the bile ducts. The production and also secretion that bile require active transport systems within hepatocytes and cholangiocytes in addition to a structurally and functionally undamaged biliary tree. Initially, hepatocytes develop bile by secreting conjugated bilirubin, bile salts, cholesterol, phospholipids, proteins, ions, and also water into their canaliculi (thin tubules between adjacent hepatocytes that eventually join to form bile ducts) <1>. The canalicular membrane of the hepatocyte is the key bile secretory device that contains the intracellular organelles, the cytoskeleton the the hepatocyte, and carrier proteins. The transport proteins in the canalicular membrane transport bile acid and also ions. Transporter proteins uncovered within the canalicular membrane use energy to secrete molecules into bile against concentration gradients. Through this energetic transport, osmotic and electrochemical gradients are formed. When conjugated bile salts enter the canaliculus, water adheres to by osmosis. The electrochemical gradient allows for the passive diffusion of not natural ions such together sodium. The most far-ranging promoter the bile development is the i of conjugated bile salts into the biliary canaliculus. The complete bile flow in a day is roughly 600 ml, of i beg your pardon 75% is derived from hepatocytes, and also 25% is native cholangiocytes. Approximately half of the hepatocyte component of bile circulation (about 225 ml per day) is bile salt-dependent, and the remaining half bile salt independent. Osmotically energetic solutes such together glutathione and bicarbonate encourage bile salt elevation bile flow <5><6>.

Canaliculi north bile into ductules or cholangioles or canals of Hering. The ductules connect with interlobular bile ducts, which are accompanied through branches that the portal vein and also hepatic artery developing portal triads. Bile is consequently modified through ductular epithelial cells together it passes v the biliary tree. This cells, well-known as cholangiocytes, dilute, and alkalinize the bile with hormone-regulated absorptive and also secretory processes. The cholangiocytes have actually receptors that modulate the bicarbonate-rich ductular bile flow, i m sorry is regulated by hormones. This receptors encompass receptors because that secretin, somatostatin, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride-bicarbonate exchanger. For example, once secretin stimulates receptor in the cholangiocyte, a cascade is initiated, i beg your pardon activates the CFTR chloride channel and allows the exchange of bicarbonate for chloride. In contrast, somatostatin inhibits the cAMP synthesis within the cholangiocytes, causing the the opposite effect. If bombesin, vasoactive intestinal polypeptide, acetylcholine, and also secretin boost bile flow, somatostatin, gastrin, insulin, and endothelin inhibit the flow <7>.

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Bile Acids

Cholesterol catabolism through hepatocytes outcomes in the synthetic of the two major primary bile acids, cholic acid and chenodeoxycholic acid. This procedure involves multiple steps, with cholesterol 7alpha-hydroxylase acting as the rate-limiting enzyme. Major bile mountain undergo dehydroxylation by bacteria in the tiny intestine, creating the secondary bile acids deoxycholic acid and also lithocholic acid, respectively. Both main and secondary bile acids are conjugated by the liver v an amino acid, either glycine or taurine. Conjugated bile acids are well-known as bile salts. Bile salt inhibit cholesterol 7alpha-hydroxylase, to decrease the synthesis of bile acids. In spite of the raised water solubility the bile salts, they room amphipathic molecules overall <8>. This critical property allows them to successfully emulsify lipids and form micelles with the assets of lipid digestion. The bile acid pool is maintained largely via the enterohepatic circulation and, to a small extent (about 5%), by the hepatic synthesis of bile acids, as lengthy as the day-to-day fecal ns of bile acids do not exceed 20% of the pool.