Physiology, Bile Secretion
Bile is a physiological aqueous solution produced and secreted by the liver. It consists mainly of bile salts, phospholipids, cholesterol, conjugated bilirubin, electrolytes, and water . Bile travels through the liver in a series of ducts, eventually exiting through the common hepatic duct. Bile flows through this duct into the gallbladder, where it is concentrated and stored. When stimulated by the hormone cholecystokinin (CCK), the gallbladder contracts, pushing bile through the cystic duct and into the common bile duct. Simultaneously, the sphincter of Oddi relaxes, permitting bile to enter the duodenal lumen. The hormone secretin also plays an important role in the flow of bile into the small intestine. By stimulating biliary and pancreatic ductular cells to secrete bicarbonate and water in response to the presence of acid in the duodenum, secretin effectively expands the volume of bile entering the duodenum. In the small intestine, bile acids facilitate lipid digestion and absorption. Only approximately 5% of these bile acids are eventually excreted. The majority of bile acids are efficiently reabsorbed from the ileum, secreted into the portal venous system, and returned to the liver in a process known as enterohepatic recirculation .
Bile is produced by hepatocytes and it is then modified by the cholangiocytes lining the bile ducts. The production and secretion of bile require active transport systems within hepatocytes and cholangiocytes in addition to a structurally and functionally intact biliary tree. Initially, hepatocytes produce bile by secreting conjugated bilirubin, bile salts, cholesterol, phospholipids, proteins, ions, and water into their canaliculi (thin tubules between adjacent hepatocytes that eventually join to form bile ducts) . The canalicular membrane of the hepatocyte is the main bile secretory apparatus that contains the intracellular organelles, the cytoskeleton of the hepatocyte, and carrier proteins. The carrier proteins in the canalicular membrane transport bile acid and ions. Transporter proteins found within the canalicular membrane use energy to secrete molecules into bile against concentration gradients. Through this active transport, osmotic and electrochemical gradients are formed. When conjugated bile salts enter the canaliculus, water follows by osmosis. The electrochemical gradient allows for the passive diffusion of inorganic ions such as sodium. The most significant promoter of bile formation is the passage of conjugated bile salts into the biliary canaliculus. The total bile flow in a day is approximately 600 ml, of which 75% is derived from hepatocytes, and 25% is from cholangiocytes. Approximately half of the hepatocyte component of bile flow (about 225 ml per day) is bile salt-dependent, and the remaining half bile salt independent. Osmotically active solutes such as glutathione and bicarbonate promote bile salt independent bile flow .
Canaliculi empty bile into ductules or cholangioles or canals of Hering. The ductules connect with interlobular bile ducts, which are accompanied by branches of the portal vein and hepatic artery forming portal triads. Bile is subsequently modified by ductular epithelial cells as it passes through the biliary tree. These cells, known as cholangiocytes, dilute, and alkalinize the bile through hormone-regulated absorptive and secretory processes. The cholangiocytes have receptors that modulate the bicarbonate-rich ductular bile flow, which is regulated by hormones. These receptors include receptors for secretin, somatostatin, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride-bicarbonate exchanger. For example, when secretin stimulates receptors in the cholangiocyte, a cascade is initiated, which 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 opposite effect. While bombesin, vasoactive intestinal polypeptide, acetylcholine, and secretin enhance bile flow, somatostatin, gastrin, insulin, and endothelin inhibit the flow .
Cholesterol catabolism by hepatocytes results in the synthesis of the two major primary bile acids, cholic acid and chenodeoxycholic acid. This process involves multiple steps, with cholesterol 7alpha-hydroxylase acting as the rate-limiting enzyme. Primary bile acids undergo dehydroxylation by bacteria in the small intestine, forming the secondary bile acids deoxycholic acid and lithocholic acid, respectively. Both primary and secondary bile acids are conjugated by the liver with an amino acid, either glycine or taurine. Conjugated bile acids are known as bile salts. Bile salts inhibit cholesterol 7alpha-hydroxylase, decreasing the synthesis of bile acids. Despite the increased water solubility of bile salts, they are amphipathic molecules overall . This critical property allows them to effectively emulsify lipids and form micelles with the products of lipid digestion. The bile acid pool is maintained mainly via the enterohepatic circulation and, to a small extent (about 5%), by the hepatic synthesis of bile acids, as long as the daily fecal loss of bile acids do not exceed 20% of the pool.
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The main steps in the formation of bile are the uptake of bile acids and ions from plasma across the basolateral (sinusoidal) membrane, transport through the hepatocyte, and excretion via the canalicular membrane.
The sodium-potassium ATPase on the basolateral membrane of the hepatocyte maintains sodium and potassium gradients. Because three sodium ions are expelled from the cell in return for receiving two potassium ions, an electrochemical gradient is formed . The relative negative charge inside the hepatocyte favors the uptake of positively charged ions, while the sodium gradient fuels the sodium-dependent taurocholate cotransporter protein. This transporter allows for the uptake of conjugated bile acids. In contrast, the organic anion transporter protein does not require sodium to import organic anions. There are several other transporters found on the basolateral surface of the hepatocyte, including the sodium-taurocholate co-transporting protein, ion exchangers that regulate pH, such as the sodium-hydrogen exchanger and the sodium-bicarbonate cotransporter, organic anion and cation transporter, and non-esterified fatty acid transporters.
The transporter proteins found in the canalicular membrane are primarily members of the ATP-binding cassette protein family . These proteins use active transport to secrete molecules and enzymes into the bile. These transporter proteins include the bile salt export pump (BSEP), the multispecific organic anion transporter (MRP2), the multiple drug resistance 1 and 3 (MDR1 and MDR3), ATP dependent phospholipid transporter (flippase), ATP dependent transporter of organic cations, and the canalicular bicarbonate transporter . The canalicular membrane transporters help secrete molecules into bile against concentration gradients, and also enzymes such as alkaline phosphatase. Contractile microfilaments facilitate the secretion of bile through the canaliculi. The canalicular membrane accounts for only 1% of the surface area of the hepatocyte.
In normal development, the synthesis of bile acids first occurs during weeks 5 to 9 of gestation, with bile secretion occurring at 12 weeks of gestation, and surging after 17 weeks gestation  . After birth, the composition of bile acids further changes; in the neonatal period, the ratio of cholic to chenodeoxycholic acid is approximately 2.5, whereas this changes to approximately 1.6 in the adult .
Abnormal development of the biliary tree can cause congenital liver disease. These primary cholangiopathies include ductopenic syndromes, ductal plate malformation syndromes, polycystic liver diseases, and fibro-polycystic liver diseases .
Bilirubin, the major pigment of bile, is an end product of heme catabolism that travels to the liver bound to albumin. Once inside the liver, the enzyme uridine diphosphate glucuronyltransferase (UDPGT) conjugates bilirubin to form bilirubin glucuronide. The water-soluble conjugated bilirubin is then secreted into bile, providing its characteristic yellow color .
Liver: Site of bile formation, reuptake of bile acids, and reuptake of urobilinogen
Bile ducts: Modify and transport bile, secrete ions and water into bile
Gallbladder: Stores and concentrates bile
- Bacteria form secondary bile acids via dehydroxylation of primary bile acids
- Bilirubin glucuronide is converted back to bilirubin
- Bacteria convert bilirubin to urobilinogen
Duodenum: Site of lipid digestion and absorption facilitated by bile
Ileum: Site of reabsorption of bile salts
Portal circulation: Transports reabsorbed bile salts back to the liver
Rectum: Urobilin and stercobilin (compounds oxidized from urobilinogen) are responsible for dark fecal pigment.
Some urobilinogen is excreted in the urine .
The main functions of bile are 2-fold:
- To facilitate lipid absorption and digestion
- To eliminate waste products from the body
Lipid Absorption and Digestion
Through the process of emulsification, bile acids break down large lipid droplets into smaller ones, increasing the surface area for digestive enzymes. Emulsification is possible due to the amphipathic property of bile salts . The hydrophilic portion of the bile salts surrounds the lipid, forcing the lipid to disperse as the negative charges repel each other. Bile salts also allow the products of lipid digestion to be transported as micelles. The core of the micelle contains monoglycerides, lysolecithin, fatty acids, and the hydrophobic portion of the bile salt. The hydrophilic portion of the bile salt surrounds the lipid core, increasing solubility. Without bile salts, the fat-soluble vitamins (A, D, E, K) cannot be absorbed.
Elimination of Waste Products
Cholesterol is eliminated through its conversion into bile acids, allowing the body to maintain cholesterol homeostasis. Bile acid sequestrants, medications intended to lower cholesterol, function by binding bile acids in the small intestine, and increasing their excretion in the stool. Bilirubin is also eliminated through its secretion into bile, where it eventually forms the dark pigment of feces .
The decrease or cessation of bile formation or flow is known as cholestasis. Cholestasis can result from the impaired canalicular secretion of bile, ductular disease, or obstruction of bile flow through the biliary tree. Causes of decreased canalicular secretion include drugs, sex hormones, and inherited defects. Ductal diseases include primary biliary cirrhosis and primary sclerosing cholangitis. Bile duct obstruction is most commonly due to gallstones but is also seen with cancers of the bile duct or pancreas .
Clinically, symptoms of cholestasis include pruritus, dark urine, pale stools, and steatorrhea. Like bilirubin, other substances that are normally excreted in bile, such as gamma-glutamyl transferase, alkaline phosphatase, and cholesterol, accumulate in the blood. Fat malabsorption may lead to deficiencies of vitamin A, D, E, and K. On examination, non-tender hepatomegaly and scratch marks on the skin due to pruritus may be present. A careful history and exam with appropriate diagnostic testing are necessary to narrow the differential diagnosis of cholestasis and create the proper treatment plan .
The principal therapeutic options for symptomatic management of cholestasis are with ursodeoxycholic acid, a hydrophilic bile acid, and cholestyramine, a bile acid sequestrant  . However, new and emerging therapies, such as gene therapies, hepatocyte transplants, and stem cell infusions, are being developed to improve the treatment of congenital cholestatic disorders with poor morbidity and mortality rates .
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