What is the organ responsible for the production of bile

The Anatomy of the Biliary System

System Responsible for Bile Secretion, Storage, and Transportation

Jane Kim, MD, is board-certified in internal medicine and works as a medical editor, writer, and consultant.

The biliary system, also called the biliary tract or biliary tree, is a system of ducts (narrow tubular structures), organs (including the liver, gallbladder, and pancreas), and associated structures that function to produce, store, secrete, and transport bile.

Bile is a greenish-brown, thick substance produced in the liver and stored in the gallbladder. Bile’s function is to be released into the small intestine when a fatty meal is ingested to help break down fats for absorption.

Once food has gone through the initial process of digestion in the stomach, it moves into the duodenum (the first segment of the small intestine). Bile and other digestive secretions travel into the duodenum via the biliary tract’s system of bile ducts. These continue the digestive process by breaking down food so its nutrients can be absorbed.

Terms

To fully understand how the biliary system works, it’s important to know the definition of some related medical terms, including the following.

Duodenum: This is the first of three sections of the small intestine, and receives food from the stomach and digestive juices from the liver, gallbladder, and pancreas via the biliary tract. This is the part of the small intestine that is primarily involved in breaking down food so that nutrients can later be absorbed in the jejunum (middle section of the small intestine).

Liver: A large glandular organ that performs many vital metabolic functions, such as the digestion of fats to make energy in the body. The liver cells make bile.

Bile: A thick, greenish-brown substance made in the liver and stored in the gallbladder, bile is comprised of water, bile acids, cholesterol, phospholipids, bile pigments (such as bilirubin), and electrolytes. It is important in enabling the body to digest and absorb fats and fat-soluble vitamins, such as vitamins D and K.

Bile duct: This is a small, hollow tube that functions to transport bile. The biliary system is comprised of a system of these ducts, which flow from the liver to the gallbladder for storage and then into the small intestine (duodenum).

Gallbladder: A pear-shaped organ located in front of the duodenum, just underneath the liver, the gallbladder’s main function is to store bile. It connects to the cystic duct.

Pancreas: A large gland located behind the stomach, the pancreas secretes pancreatic enzymes (such as lipase, which breaks down fats) into the biliary system via the pancreatic duct.

Gallstone: Abnormal, small, hard masses comprised of bile pigments, cholesterol, and calcium salts, gallstones can cause a blockage of bile ducts, a condition called cholestasis.

Biliary System Anatomy

The organs, ducts, and other structures of the biliary system are located in the upper-right abdominal quadrant, while the gallbladder is located just below the liver.

Connected to the liver and gallbladder are the extrahepatic ducts, located outside of the liver, which function to transport bile. Note that some bile ducts are also located inside the liver. These function to drain bile out of the organ and are called intrahepatic ducts.

Structure

The biliary system is comprised of a series of ducts, organs, and other structures responsible for producing, storing, and transporting bile. The bile is made in the cells of the liver and travels to the gallbladder to be stored for later use.

When a fatty meal is ingested, the bile is released and travels to the small intestine through this system of ducts to its final destination, the duodenum.

Bile Flow Through the Biliary System

Through the system of ducts and other structures of the biliary system, bile travels in a controlled manner.

Anatomical Variations

Aberrant ducts are a common variation from the normal anatomy that comprises the biliary system. Aberrant ducts are not anatomically structured the way they should be. For example, the ducts may abnormally join the wrong ducts, so that bile does not flow properly.

In fact, according a study published in Liver and Biliary Tract Surgery, “50% of patients presenting with gallbladder stones or common bile duct stones show a significant variation from what is generally considered as the expected normal pattern.”

A 2011 study discovered as many as 22 variations of bile ducts in 59.5% of the study participants who had liver surgery. These included an extra right hepatic duct (in which a cystic duct drained) and five other abnormalities that had never been described before.

Variation from the normal anatomy of the bile ducts is a primary reason the ducts get inadvertently injured during some types of surgery.

Function of the Biliary System

There are three important functions of the biliary system:

Associated Conditions

Biliary disease describes any condition that affects the gallbladder, bile ducts, and other structures needed to produce and transport bile. Common maladies of the biliary system include gallbladder disease, biliary colic, and bile duct obstruction.

Gallbladder Disease

Gallstones are the most common gallbladder condition, but tumors and acute acalculous cholecystitis (sudden, severe inflammation of the gallbladder without gallstones) are other common types of biliary disease.

Biliary Colic

Biliary colic is intermittent pain in the upper-right quadrant of the abdomen or above the stomach (epigastrium). Caused by a temporary obstruction of the cystic duct (this is usually secondary to a gallstone that is trapped in the cystic duct), the pain resulting from this condition can range from mild to severe.

If the obstruction is not removed or the gallstone doesn’t pass on its own, it results in cholecystitis (an acute inflammation of the gallbladder).

Bile Duct Obstruction

Also known as biliary obstruction, this is the blockage of any of the ducts in the biliary system. This condition most commonly occurs from a gallstone, but can also be caused by a tumor or another underlying cause.

Treatment

Treatment for biliary disease may include:

Tests

Several types of tests are done to diagnose abnormalities and diseases of the biliary system.

Frequently Asked Questions

Bile is a natural substance produced by the liver that helps break down fats in the small intestine. The breakdown of fats allows for their nutrients to be absorbed by the intestines.

The common hepatic duct assists in the transport of bile from the liver to the intestine. It receives bile from the left and right hepatic ducts, and then joins with the cystic duct to form the common bile duct. From there, bile is released into the small intestine.

The liver is located mainly on the upper right side of the abdomen and reaches across the upper abdomen. It sits directly below the lungs and above the stomach, pancreas, and gallbladder.

Medical University of South Carolina (MUSC). Biliary tree.

Karaliotas CC, Papaconstantinou T, Karaliotas CC. Anatomical variations and anomalies of the biliary tree, veins and arteries. in: Liver and Biliary Tract Surgery. ISBN:978-3-211-49275-8.

George Washington Hospital. Biliary disease.

What is the organ responsible for the production of bile

The primary digestive function of bile is to aid in the dispersion and digestion of fat in the lumen of the small intestine. Bile is formed initially in the hepatocyte (liver cell), and the rate of formation is dependent primarily on the rate at which bile acids are secreted into the bile channels, or canaliculi. A portion of the bile flow, however, is related to factors other than the secretion of bile acids; in particular, it appears to be dependent on the secretion of sodium from the hepatocyte and is also partially governed by the action of intestinal hormones such as secretin, cholecystokinin (CCK), and gastrin. The total bile acid pool at any one time measures about 3 grams (about 0.1 ounce), almost all of which is contained at rest in the gallbladder. In its passage through the biliary tract, hepatic bile is concentrated to as little as one-tenth of its original volume by the selective reabsorption of water, chloride, and bicarbonate. This concentration process takes place largely in the gallbladder, and, as a result, bile from this organ is much thicker in density and darker in colour (owing to the concentration of pigments) than is bile emerging from the liver. Distension of the duodenum, particularly by a meal containing fat, provokes the secretion of CCK, a hormone that causes contractions of the muscular layer in the wall of the gallbladder.

Aside from inorganic ions (sodium, potassium, calcium, magnesium, chloride, and bicarbonate), bile contains protein and bilirubin; the latter is responsible for its golden colour in dilute solutions and dark amber colour in concentrate. It is richest, however, in bile acids (derived from cholesterol in the hepatocyte), phospholipids (largely phosphatidyl choline, or lecithin), and cholesterol. Cholesterol is a four-ringed sterol that is absorbed from the diet or synthesized by the liver and the intestinal lining. Normally not soluble in watery secretions, cholesterol is carried in a colloidal solution in bile in the form of mixed aggregates of complexes containing bile acids and lecithin. In the absence of adequate amounts of lecithin and bile acids, cholesterol crystallizes. The liver synthesizes two types of primary bile acid from cholesterol, called chenodeoxycholic acid and cholic acid. In the lower intestine bacterial action removes one of the hydroxyl groups (dehydroxylation) from cholic acid, changing it to deoxycholic acid. This secondary bile acid appears in bile because it is absorbed from the intestine and recirculated to the liver. Chenodeoxycholic acid is also dehydroxylated in the intestine, becoming lithocholic acid, a small amount of which is also reabsorbed and appears in normal bile.

Pancreas

The pancreas is a long, narrow gland that is situated transversely across the upper abdomen, behind the stomach and the spleen. The midportion of the pancreas lies against the vertebral column, the abdominal aorta, and the inferior vena cava.

The pancreas is both an exocrine (ductal) and endocrine (ductless) gland. The exocrine tissue, called acinar tissue, produces important digestive enzyme precursors that are transmitted into the small intestine, while the endocrine tissue (contained in the islets of Langerhans) produces at least two hormones (insulin and glucagon) that are important in the regulation of carbohydrate metabolism. Two other hormones produced by the pancreas, vasoactive intestinal polypeptide and somatostatin, are pivotal elements in the control of intestinal secretion and motility.

Individual acinar cells have the shape of a truncated pyramid, arranged in groups around a central ductal lumen. These central ducts empty into progressively larger intercalated and collecting ducts that eventually join the pancreatic duct (duct of Wirsung). The pancreatic duct in turn enters the hepatopancreatic ampulla (ampulla of Vater) of the duodenum, where, in about 80 percent of instances, it is joined by the common bile duct. Occasionally the junction with the common bile duct is proximal to the ampulla, and in a few cases the pancreatic duct and the common bile duct join the duodenum separately.

Acinar cells

The acinar cells constitute more than 95 percent of the cellular population of the exocrine pancreas. They produce a variety of digestive proteins, or enzymes, involved principally with the degradation of dietary proteins (proteases), fats (lipases), and carbohydrates (amylases) in the intestine. Other protein secretions include a trypsin inhibitor, a so-called “stone protein” that keeps calcium in solution, and various serum proteins, including albumin and immunoglobulins.

In the acinar cells almost all enzymatic proteins are synthesized on ribosomes from amino acids carried to the pancreas by the bloodstream. Enzyme precursors are conjugated in the Golgi apparatus and then concentrated into membrane-wrapped zymogen granules, which are stored in the cytoplasm before secretion. Enzymatic secretion is mediated by stimulants such as secretin, a hormone released from the duodenum by the introduction of gastric acid, cholecystokinin (CCK), released by the presence of dietary fat, amino acids, hydrochloric acid, and acetylcholine, which is produced as a response to the sensory aspects of feeding and to the physical effects of chewing and swallowing. Upon binding of specific receptor sites on the acinar membrane with CCK or acetylcholine, the zymogen granules migrate to the apex of the acinar cell, where they are extruded into the central ductal lumen. Binding of vasoactive intestinal polypeptide or secretin to acinar receptors causes increased production of bicarbonate, sodium, water, and enzymes by acinar cells and small ductal cells. Bicarbonate is secreted in exchange for chloride, and sodium is exchanged for hydrogen, with a resultant increased acidity of the blood leaving the actively secreting pancreas. Binding of CCK causes production of bicarbonate and enzymes by the acinar cells.

In the absence of CCK and acetylcholine, as in fasting subjects or in patients being fed intravenously, the synthesis of zymogen by the acinar cells is markedly reduced. Pancreatic atrophy also occurs after removal of the pituitary gland, probably owing to the absence of growth hormone. Thus, CCK, acetylcholine, and growth hormone are pancreatotrophic, or pancreas-feeding, hormones. The pancreas itself also appears to secrete an as-yet-unidentified hormone that is trophic, or nutritive, to the liver.

What is the organ responsible for the production of bile

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bile, also called gall, greenish yellow secretion that is produced in the liver and passed to the gallbladder for concentration, storage, or transport into the first region of the small intestine, the duodenum. Its function is to aid in the digestion of fats in the duodenum. Bile is composed of bile acids and salts, phospholipids, cholesterol, pigments, water, and electrolyte chemicals that keep the total solution slightly alkaline (with a pH of about 7 to 8). Bile is continually secreted from the cells of the liver into the common bile duct and gallbladder; once in the gallbladder it is usually concentrated to about 5 times—and sometimes as high as 18 times—the strength of the original secretion. The amount of bile secreted into the duodenum is controlled by the hormones cholecystokinin, secretin, gastrin, and somatostatin and also by the vagus nerve. About 800 to 1,000 ml of bile (before concentration) are produced daily by the liver.

Bile salts are composed of the salts of four different kinds of free bile acids (cholic, deoxycholic, chenodeoxycholic, and lithocholic acids); each of these acids may in turn combine with glycine or taurine to form more complex acids and salts. Bile salts and acids can be synthesized from cholesterol or extracted from the bloodstream by the liver. They pass from the liver into the small intestine, where they act as detergents to emulsify fat and reduce the surface tension on fat droplets to prepare them for the action of pancreatic and intestinal fat-splitting enzymes. The salts are large, negatively charged ions that are not readily absorbed by the upper region of the small intestine; consequently, they remain in the small intestine until most of the fat is digested. In the lower small intestine, the salts and acids are absorbed and passed back into the bloodstream until they are once again extracted by the liver; this cycle, from the liver to the small intestine and blood and then back to the liver, is called enterohepatic circulation. Some salts and acids are lost during this process; these are replaced in the liver by continual synthesis from cholesterol. The rate of synthesis is directly related to the amount of acids and salts lost. Bile salts do not normally reach the colon; when they do, however, they may inhibit the absorption of water and sodium, causing a watery diarrhea.

Bile salts and acids are transported in a fluid that contains water, sodium, chloride, and bicarbonates. This fluid is produced in the liver, and it serves to neutralize hydrochloric acid passed from the stomach into the small intestine. Water-insoluble wastes that the liver removes from blood, such as cholesterol, steroids, drugs, and hemoglobin pigments, are carried in the fluid to the excretory system. Hemoglobin pigments are broken down, producing several bile fluid compounds, including bilirubin, which has no known function other than that of a colouring agent. Traces of other substances can also be found in bile including mucus, serum proteins, lecithin, neutral fats, fatty acids, and urea.

This article was most recently revised and updated by Kara Rogers.

Bile Production

Original Author(s): Alliya Ghanchi
Last updated: 19th May 2022
Revisions: 14

Original Author(s): Alliya Ghanchi
Last updated: 19th May 2022
Revisions: 14

Bile is an aqueous, alkaline, greenish-yellow liquid whose main function is to emulsify fats in the small intestine and to eliminate substances from the liver. The liver produces 0.25-1L of bile per day.

In this article, we shall consider the many different components of bile, and how these are produced in the liver.

Constituents of Bile

Bile is made up of bile acids, cholesterol, phospholipids, bile pigments (such as bilirubin and biliverdin), electrolytes and water.

These are split into two main groups, known as the bile acid-dependent and bile acid-independent components:

Both the dependent and independent components of bile enter the intrahepatic bile ducts which drain into the biliary tree, a series of ducts which transport bile from the liver to the gallbladder and duodenum.

Bile is continuously produced, but we only need it during and after meals. Therefore the gallbladder concentrates and stores bile, by removing the water and ions. After eating, the hormone cholecystokinin is released from the duodenum. This stimulates gallbladder contraction and relaxes the sphincter of Oddi, thus allowing bile to flow into the duodenum.

Fig 1 – Diagram to show the position of the bile duct

Bile Acids and Bile Salts

There are 2 primary bile acids:

When these bile acids are conjugated with the amino acids glycine and taurine, they form bile salts. Bile salts are more soluble than bile acids and act as detergents to emulsify lipids.

Bile salts are amphipathic which means they have a hydrophobic end which is lipid-soluble and a hydrophilic end which is water-soluble. This structure allows bile salts to emulsify fats into smaller droplets, increasing the surface area for lipids to be broken down by duodenal lipases.

The bile acids coat the products of lipid breakdown as well as cholesterol and phospholipids to form spherical structures known as micelles. Micelles play an important role in the digestion of fats and transport their contents to the intestinal epithelium where they can be absorbed. However, the bile acids don’t enter the gut epithelial cells with the lipids, rather they recirculate back to the liver, via the enterohepatic circulation.

Bile Pigments

Bile also contains bile pigments which are excretory products of the liver and include biliverdin and bilirubin. Bilirubin is a breakdown product of haemoglobin and is conjugated in the liver and secreted into bile. Bile pigments are normally excreted in the faeces and make the faeces appear brown. Problems in the liver or biliary tree often result in the accumulation of bilirubin in the blood, which manifests as jaundice.

Enterohepatic Circulation of Bile Acids

Bile passes out of the liver through the bile ducts and is concentrated and stored in the gallbladder. During and after a meal, bile is excreted from the gall bladder by contraction and passes into the duodenum through the common bile duct. Most of the bile acids are reabsorbed in the terminal ileum and returned to the liver via the hepatic portal vein. The liver then extracts the bile salts.

The enterohepatic circulation allows the liver to recycle and preserve a pool of bile acids.

Clinical Relevance

Gallstones

Gallstones are small lumps of solid material that form in the gall bladder. Abnormal concentrations of bile acids can increase the risk of precipitation of bile out of solution to form stones. They can be made up entirely of cholesterol, bile pigments or a mixture of the two. Risk factors for gallstone formation include being female, obesity, age >40, and poor diet.

Gallstones are often asymptomatic but can cause painful biliary colic if they move to occlude the neck of the gall bladder or the biliary tree. Biliary colic is intermittent pain in the right upper quadrant, typically precipitated by the consumption of a fatty meal and subsequent contractions of the gallbladder. This can lead to further complications such as cholecystitis (inflammation of the gallbladder) or ascending cholangitis (inflammation of the bile ducts). Associated symptoms include nausea/vomiting, abdominal pain, jaundice and fever.

Fig 2 – Diagram demonstrating a gallstone blocking the bile duct

Jaundice

Jaundice is the yellowish pigmentation of the skin and eyes due to excessive bilirubin levels in the blood. There are many different causes for jaundice, and these can be split into three categories:

Steatorrhoea

Steatorrhoea is another symptom that can indicate dysfunctional bile production. If bile acids/salts or pancreatic lipases are not secreted in adequate amounts, less fat emulsification and absorption occurs which causes fat to appear in the faeces. The faeces appear pale, floating and foul-smelling, which is known as steatorrhoea.

Try again to score 100%. Use the information in this article to help you with the answers.

Bile is an aqueous, alkaline, greenish-yellow liquid whose main function is to emulsify fats in the small intestine and to eliminate substances from the liver. The liver produces 0.25-1L of bile per day.

In this article, we shall consider the many different components of bile, and how these are produced in the liver.

Constituents of Bile

Bile is made up of bile acids, cholesterol, phospholipids, bile pigments (such as bilirubin and biliverdin), electrolytes and water.

These are split into two main groups, known as the bile acid-dependent and bile acid-independent components:

Both the dependent and independent components of bile enter the intrahepatic bile ducts which drain into the biliary tree, a series of ducts which transport bile from the liver to the gallbladder and duodenum.

Bile is continuously produced, but we only need it during and after meals. Therefore the gallbladder concentrates and stores bile, by removing the water and ions. After eating, the hormone cholecystokinin is released from the duodenum. This stimulates gallbladder contraction and relaxes the sphincter of Oddi, thus allowing bile to flow into the duodenum.

Bile Acids and Bile Salts

There are 2 primary bile acids:

When these bile acids are conjugated with the amino acids glycine and taurine, they form bile salts. Bile salts are more soluble than bile acids and act as detergents to emulsify lipids.

Bile salts are amphipathic which means they have a hydrophobic end which is lipid-soluble and a hydrophilic end which is water-soluble. This structure allows bile salts to emulsify fats into smaller droplets, increasing the surface area for lipids to be broken down by duodenal lipases.

The bile acids coat the products of lipid breakdown as well as cholesterol and phospholipids to form spherical structures known as micelles. Micelles play an important role in the digestion of fats and transport their contents to the intestinal epithelium where they can be absorbed. However, the bile acids don’t enter the gut epithelial cells with the lipids, rather they recirculate back to the liver, via the enterohepatic circulation.

Bile Pigments

Bile also contains bile pigments which are excretory products of the liver and include biliverdin and bilirubin. Bilirubin is a breakdown product of haemoglobin and is conjugated in the liver and secreted into bile. Bile pigments are normally excreted in the faeces and make the faeces appear brown. Problems in the liver or biliary tree often result in the accumulation of bilirubin in the blood, which manifests as jaundice.

Enterohepatic Circulation of Bile Acids

Bile passes out of the liver through the bile ducts and is concentrated and stored in the gallbladder. During and after a meal, bile is excreted from the gall bladder by contraction and passes into the duodenum through the common bile duct. Most of the bile acids are reabsorbed in the terminal ileum and returned to the liver via the hepatic portal vein. The liver then extracts the bile salts.

The enterohepatic circulation allows the liver to recycle and preserve a pool of bile acids.

Clinical Relevance

Gallstones

Gallstones are small lumps of solid material that form in the gall bladder. Abnormal concentrations of bile acids can increase the risk of precipitation of bile out of solution to form stones. They can be made up entirely of cholesterol, bile pigments or a mixture of the two. Risk factors for gallstone formation include being female, obesity, age >40, and poor diet.

Gallstones are often asymptomatic but can cause painful biliary colic if they move to occlude the neck of the gall bladder or the biliary tree. Biliary colic is intermittent pain in the right upper quadrant, typically precipitated by the consumption of a fatty meal and subsequent contractions of the gallbladder. This can lead to further complications such as cholecystitis (inflammation of the gallbladder) or ascending cholangitis (inflammation of the bile ducts). Associated symptoms include nausea/vomiting, abdominal pain, jaundice and fever.

Jaundice

Jaundice is the yellowish pigmentation of the skin and eyes due to excessive bilirubin levels in the blood. There are many different causes for jaundice, and these can be split into three categories:

Steatorrhoea

Steatorrhoea is another symptom that can indicate dysfunctional bile production. If bile acids/salts or pancreatic lipases are not secreted in adequate amounts, less fat emulsification and absorption occurs which causes fat to appear in the faeces. The faeces appear pale, floating and foul-smelling, which is known as steatorrhoea.

Bile Function: Biliary Composition, Role of Bile Acids and Biliary Secretion

It is a yellow-green fluid produced in the liver, stored in the gallbladder, and passed through the common bile duct to the duodenum, where it helps digest fat.

Functions of bile

There are two fundamentally essential functions of bile in all species:

Bile does not contain enzymes like other secretions of the gastrointestinal tract. Instead, it has bile salts (acids) that can:

Adult humans produce 400 to 800 ml of bile daily, and other animals have proportionally similar amounts. The secretion of bitterness can be considered to occur in two stages:

In species with gallbladder (man and most domestic animals, except horses and rats), there is an additional modification of bile in that organ.

The gallbladder stores and concentrates bile during the fasting state. Typically, bile is concentrated five times in the gallbladder by absorbing water and small electrolytes: virtually all organic molecules are retained.

Secretion in the bile is an essential route to eliminating cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bitterness, it becomes soluble by bile acids and lipids such as lecithin.

Gallstones, most of which are predominantly cholesterol, result from processes that allow cholesterol to precipitate out of the solution in the bile.

Biliary composition

Bile, either from the liver or gallbladder, contains the following substances:

As mentioned, bile from the gallbladder is concentrated compared to hepatic bile. Bile salts are the most significant volume of bile in the gallbladder and can be six times more concentrated than bile salts in the hepatic bile.

Hepatic bile, however, has higher concentrations of:

Role of bile acids in the digestion and absorption of fat

Bile acids are derivatives of cholesterol synthesized in the hepatocyte.

Cholesterol, ingested as part of the diet or derivative of the hepatic synthesis, is converted into bile acid, cholic acid, and chenodeoxycholic acid, which is then conjugated with an amino acid ( glycine or taurine) to produce the conjugated form actively secreted in the canaliculus.

Bile acids are facial amphipathic; they contain both hydrophobic (lipid-soluble) and polar (hydrophilic) faces.

The cholesterol-derived portion of a bile acid has a hydrophobic face (that with methyl groups) and one that is hydrophilic (that with the hydroxyl groups); the amino acid conjugate is polar and hydrophilic.

Its amphipathic nature allows bile acids to perform two essential functions:

Emulsification of lipid aggregates: bile acids have detergent action on the fat particles of the diet that causes the fat globules to decompose or emulsify into tiny microscopic droplets.

Emulsification is not digestion per se, but it is vital because it dramatically increases the surface area of ​​the fat, making it available for digestion by lipases, which can not access the interior of the lipid droplets.

Solubilization and transport of lipids in an aqueous environment: bile acids are lipid transporters. They can solubilize many lipids forming micelles (aggregates of lipids such as fatty acids, cholesterol, and monoglycerides) that remain suspended in water.

Bile acids are also critical for the transport and absorption of fat-soluble vitamins.

Role of bile acids in cholesterol homeostasis

Hepatic synthesis of bile acids accounts for most of the breakdown of cholesterol in the body.

Approximately 500 mg of cholesterol in humans is converted into bile acids and eliminated in the bile every day. This route for the elimination of excess cholesterol is probably important in all animals, but particularly in situations of massive cholesterol ingestion.

Interestingly, it has recently been shown that bile acids participate in the metabolism of cholesterol by functioning as hormones that alter the transcription of the speed-limiting enzyme in cholesterol biosynthesis.

Enterohepatic recirculation

Large amounts of bile acids are secreted in the intestine every day, but only relatively small parts of the body are lost. Approximately 95% of the bile acids administered to the duodenum are absorbed back into the blood within the ileum.

The venous blood of the ileum goes directly to the portal vein and, therefore, through the sinusoids of the liver. Hepatocytes extract bile acids very efficiently from sinusoidal blood, and little escapes the healthy liver in the systemic circulation.

The bile acids are transported through the hepatocytes to resect them in canaliculi. The net effect of this enterohepatic recirculation is that each bile salt molecule is reused about 20 times, often two or three times during a single digestive phase.

It should be noted that liver disease can dramatically alter this recirculation pattern; for example, diseased hepatocytes have a lower capacity to extract bile acids from the portal blood, and damage to the canalicular system can cause the exit of bile acids into the systemic circulation.

The systemic bile acid levels test is used clinically as a sensitive indicator of liver disease.

Pattern and control of biliary secretion

The bile flow is lower during fasting, and most of that is diverted to the gallbladder to concentrate.

When the chyme of an ingested food enters the small intestine, the acid, fats, and partially digested proteins stimulate the secretion of cholecystokinin and secretin.

As discussed above, these enteric hormones have essential effects on pancreatic exocrine secretion. Both are also important for the secretion and flow of bile:

Cholecystokinin: the name of this hormone describes its effect on the biliary system: colecisto = gallbladder and cynicism = movement.

The most potent stimulus for the release of cholecystokinin is the presence of fat in the duodenum. Once released, it stimulates the contractions of the gallbladder and the common bile duct, which causes the administration of bile to the intestine.

Secretin: this hormone is secreted in response to the acid in the duodenum. Its effect on the biliary system is very similar to that observed in the pancreas: it stimulates the cells of the bile ducts to secrete bicarbonate and water, which expands the volume of bile and increases its flow to the intestine.

About The Author

Dr. Cameron Troup MD

Dr. Cameron Troup,, MD is a family medicine specialist in Eastchester, NY and has been practicing for 25 years. He specializes in family medicine.