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Bile formation
Medical expert of the article
Last reviewed: 04.07.2025

The liver secretes approximately 500–600 ml of bile per day. Bile is isoosmotic to plasma and consists primarily of water, electrolytes, bile salts, phospholipids (primarily lecithin), cholesterol, bilirubin, and other endogenous or exogenous components such as proteins that regulate gastrointestinal function, drugs, or their metabolites. Bilirubin is a breakdown product of heme components during the breakdown of hemoglobin. The formation of bile salts, also known as bile acids, causes the secretion of other bile constituents, particularly sodium and water. The functions of bile salts include the excretion of potentially toxic substances (e.g., bilirubin, drug metabolites), the solubilization of fats and fat-soluble vitamins in the intestine to facilitate their absorption, and the activation of osmotic cleansing of the intestine.
The synthesis and secretion of bile require mechanisms of active transport, as well as processes such as endocytosis and passive diffusion. Bile is formed in the canaliculi between adjacent hepatocytes. Secretion of bile acids in the canaliculi is the rate-limiting step in bile formation. Secretion and absorption also occur in the bile ducts.
In the liver, bile from the intrahepatic collecting system enters the proximal, or common, hepatic duct. Approximately 50% of the bile secreted outside of meals from the common hepatic duct enters the gallbladder via the cystic duct; the remaining 50% goes directly into the common bile duct, formed by the confluence of the common hepatic and cystic ducts. Outside of meals, a small portion of bile comes directly from the liver. The gallbladder absorbs up to 90% of the water from the bile, concentrating and storing it.
Bile flows from the gallbladder into the common bile duct. The common bile duct joins with the pancreatic duct to form the ampulla of Vater, which opens into the duodenum. Before joining the pancreatic duct, the common bile duct narrows in diameter to < 0.6 cm. The sphincter of Oddi surrounds both the pancreatic and common bile ducts; in addition, each duct has its own sphincter. Bile does not normally flow retrograde into the pancreatic duct. These sphincters are highly sensitive to cholecystokinin and other gut hormones (eg, gastrin-activating peptide) and to changes in cholinergic tone (eg, due to anticholinergic agents).
During a standard meal, the gallbladder begins to contract and the bile duct sphincters relax under the influence of secreted intestinal hormones and cholinergic stimulation, which promotes the movement of approximately 75% of the gallbladder contents into the duodenum. Conversely, during fasting, the sphincter tone increases, which promotes filling of the gallbladder. Bile salts are poorly absorbed by passive diffusion in the proximal small intestine; most bile acids reach the distal ileum, where 90% are actively absorbed into the portal venous bed. Once back in the liver, bile acids are effectively extracted and quickly modified (for example, free acids are bound) and secreted back into the bile. Bile salts circulate through the enterohepatic circuit 10-12 times per day.
Anatomy of the bile ducts
Bile salts, conjugated bilirubin, cholesterol, phospholipids, proteins, electrolytes, and water are secreted by hepatocytes into the bile canaliculi. The bile secretion apparatus includes canalicular membrane transport proteins, intracellular organelles, andcytoskeletal structures. Tight junctions between hepatocytes separate the lumen of the canaliculi from the hepatic circulatory system.
The canalicular membrane contains transport proteins for bile acids, bilirubin, cations and anions. Microvilli increase its area. The organelles are represented by the Golgi apparatus and lysosomes. Vesicles are used to transport proteins (for example, IgA) from the sinusoidal to the canalicular membrane, and to deliver transport proteins synthesized in the cell for cholesterol, phospholipids and, possibly, bile acids from the microsomes to the canalicular membrane.
The cytoplasm of the hepatocyte around the tubules contains cytoskeletal structures: microtubules, microfilaments and intermediate filaments.
Microtubules are formed by polymerization of tubulin and form a network inside the cell, especially near the basolateral membrane and the Golgi apparatus, participating in receptor-mediated vesicular transport, secretion of lipids, and under certain conditions, bile acids. Microtubule formation is inhibited by colchicine.
The construction of microfilaments involves interacting polymerized (F) and free (G) actins. Microfilaments, concentrated around the canalicular membrane, determine the contractility and motility of the canals. Phalloidin, which enhances actin polymerization, and cytochalasin B, which weakens it, inhibit canal motility and cause cholestasis.
Intermediate filaments are composed of cytokeratin and form a network between plasma membranes, the nucleus, intracellular organelles, and other cytoskeletal structures. Rupture of intermediate filaments leads to disruption of intracellular transport processes and obliteration of the lumen of the tubules.
Water and electrolytes affect the composition of the tubular secretion by penetrating through the tight junctions between hepatocytes due to the osmotic gradient between the lumen of the tubules and the spaces of Disse (paracellular flow). The integrity of the tight junctions depends on the presence of the ZO-1 protein with a molecular weight of 225 kDa on the inner surface of the plasma membrane. Rupture of the tight junctions is accompanied by the entry of dissolved larger molecules into the tubules, which leads to a loss of the osmotic gradient and the development of cholestasis. Regurgitation of tubular bile into the sinusoids may be observed.
The bile canaliculi empty into ductules, sometimes called cholangiole or Hering's canals. Ductules are located mainly in portal zones and empty into interlobular bile ducts, which are the first of the bile ducts to be accompanied by branches of the hepatic artery and portal vein and are found in the portal triads. The interlobular ducts merge to form septal ducts until two main hepatic ducts are formed, emerging from the right and left lobes in the region of the porta hepatis.
[ 6 ], [ 7 ], [ 8 ], [ 9 ], [ 10 ], [ 11 ], [ 12 ], [ 13 ], [ 14 ], [ 15 ], [ 16 ]
Secretion of bile
Bile formation occurs with the participation of a number of energy-dependent transport processes. Its secretion is relatively independent of perfusion pressure. The total flow of bile in humans is approximately 600 ml/day. Hepatocytes provide the secretion of two fractions of bile: dependent on bile acids ("225 ml/day") and independent of them ("225 ml/day"). The remaining 150 ml/day are secreted by bile duct cells.
Secretion of bile salts is the most important factor in the formation of bile (the fraction dependent on bile acids). Water follows the osmotically active bile salts. Changes in osmotic activity can regulate the entry of water into bile. There is a clear correlation between the secretion of bile salts and the flow of bile.
The existence of a bile fraction independent of bile acids is demonstrated by the possibility of producing bile containing no bile salts. Thus, a continuation of bile flow is possible despite the absence of excretion of bile salts; the secretion of water is due to other osmotically active solutes such as glutathione and bicarbonates.
[ 17 ], [ 18 ], [ 19 ], [ 20 ], [ 21 ], [ 22 ], [ 23 ], [ 24 ], [ 25 ]
Cellular mechanisms of bile secretion
The hepatocyte is a polar secretory epithelial cell with basolateral (sinusoidal and lateral) and apical (tubular) membranes.
Bile formation involves the capture of bile acids and other organic and inorganic ions, their transport through the basolateral (sinusoidal) membrane, cytoplasm and canalicular membrane. This process is accompanied by osmotic filtration of water contained in the hepatocyte and paracellular space. Identification and characterization of transport proteins of the sinusoidal and canalicular membranes are complex. The study of the secretory apparatus of the canaliculi is particularly difficult, but by now a method for obtaining double hepatocytes in a short-lived culture has been developed and proven reliable in many studies. Cloning of transport proteins allows us to characterize the function of each of them separately.
The process of bile formation depends on the presence of certain carrier proteins in the basolateral and canalicular membranes. The driving force for secretion is the Na +, K + - ATPase of the basolateral membrane, providing a chemical gradient and potential difference between the hepatocyte and the surrounding space. Na +, K + - ATPase exchanges three intracellular sodium ions for two extracellular potassium ions, maintaining a concentration gradient of sodium (high outside, low inside) and potassium (low outside, high inside). As a result, the cell contents have a negative charge (–35 mV) compared to the extracellular space, which facilitates the uptake of positively charged ions and the excretion of negatively charged ions. Na +, K + -ATPase is not found in the canalicular membrane. Membrane fluidity may affect enzyme activity.
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Capture on the surface of the sinusoidal membrane
The basolateral (sinusoidal) membrane has multiple transport systems for the uptake of organic anions, which have overlapping substrate specificities. The transport proteins have been characterized previously from animal cell studies. Recent cloning of human transport proteins has provided a better understanding of their function. Organic anion transporting protein (OATP) is sodium-independent and transports a number of molecules, including bile acids, bromsulfalein, and probably bilirubin. Other transporters are also thought to transport bilirubin into the hepatocyte. Bile acids conjugated with taurine (or glycine) are transported by sodium/bile acid cotransporting protein (NTCP).
The protein that exchanges Na + /H + and regulates pH inside the cell participates in the transfer of ions across the basolateral membrane. This function is also performed by the cotransport protein for Na + /HCO 3–. The capture of sulfates, non-esterified fatty acids, and organic cations also occurs on the surface of the basolateral membrane.
[ 34 ], [ 35 ], [ 36 ], [ 37 ], [ 38 ], [ 39 ], [ 40 ]
Intracellular transport
Transport of bile acids in the hepatocyte is carried out by cytosolic proteins, among which the main role belongs to 3a-hydroxysteroid dehydrogenase. Of lesser importance are glutathione-S-transferase and proteins that bind fatty acids. The endoplasmic reticulum and the Golgi apparatus participate in the transport of bile acids. Vesicular transport is apparently activated only with a significant influx of bile acids into the cell (in concentrations exceeding physiological ones).
Transport of fluid-phase proteins and ligands such as IgA and low-density lipoproteins is accomplished by vesicular transcytosis. The transfer time from the basolateral to the canalicular membrane is about 10 min. This mechanism is responsible for only a small portion of the total bile flow and depends on the state of the microtubules.
Tubular secretion
The canalicular membrane is a specialized region of the hepatocyte plasma membrane containing transport proteins (mostly ATP-dependent) responsible for the transport of molecules into bile against the concentration gradient. The canalicular membrane also contains enzymes such as alkaline phosphatase and GGT. Glucuronides and glutathione-S-conjugates (e.g. bilirubin diglucuronide) are transported by the canalicular multispecific organic anion transporter (cMOAT), and bile acids are transported by the canalicular bile acid transporter (cBAT), whose function is partially controlled by the negative intracellular potential. Bile flow, independent of bile acids, is apparently determined by glutathione transport and also by tubular secretion of bicarbonate, possibly with the participation of Cl – /HCO 3– exchange protein.
Two enzymes of the P-glycoprotein family play an important role in the transport of substances across the canalicular membrane; both enzymes are ATP-dependent. Multidrug resistance protein 1 (MDR1) transports organic cations and also removes cytostatic drugs from cancer cells, causing their resistance to chemotherapy (hence the name of the protein). The endogenous substrate of MDR1 is unknown. MDR3 transports phospholipids and acts as a flippase for phosphatidylcholine. The function of MDR3 and its importance for the secretion of phospholipids into bile were clarified in experiments on mice lacking mdr2-P-glycoprotein (an analogue of human MDR3). In the absence of phospholipids in bile, bile acids cause damage to the biliary epithelium, ductulitis, and periductular fibrosis.
Water and inorganic ions (especially sodium) are excreted into the bile capillaries along an osmotic gradient by diffusion through negatively charged semipermeable tight junctions.
Bile secretion is regulated by many hormones and second messengers, including cAMP and protein kinase C. Increased intracellular calcium concentrations inhibit bile secretion. Bile passage through the canaliculi occurs due to microfilaments, which provide motility and contractions of the canaliculi.
Ductular secretion
Epithelial cells of the distal ducts produce a bicarbonate-rich secretion that modifies the composition of canalicular bile (the so-called ductular flow). During secretion, cAMP and some membrane transport proteins are produced, including Cl–/HCO3–exchange protein and the cysticfibrosis transmembrane conductance regulator, a membrane channel for Cl– regulated by cAMP. Ductular secretion is stimulated by secretin.
It is assumed that ursodeoxycholic acid is actively absorbed by ductular cells, exchanged for bicarbonates, recirculated in the liver and subsequently re-excreted into bile ("cholehepatic shunt"). This may explain the choleretic effect of ursodeoxycholic acid, accompanied by high biliary secretion of bicarbonates in experimental cirrhosis.
The pressure in the bile ducts, at which bile secretion occurs, is normally 15-25 cm H2O. An increase in pressure to 35 cm H2O leads to suppression of bile secretion and the development of jaundice. Secretion of bilirubin and bile acids can completely stop, and the bile becomes colorless (white bile) and resembles a mucous liquid.
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