It is a yellow-green fluid that is produced in the liver, stored in the gallbladder and passed through the common bile duct to the duodenum, where it helps digest fat.
Bile is a complex fluid that contains water, electrolytes and a battery of organic molecules that include bile acids , cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine.
Functions of bile
There are two fundamentally important functions of bile in all species:
- Bile contains bile acids, which are critical for the digestion and absorption of fat and fat-soluble vitamins in the small intestine.
- Many waste products, including bilirubin, are eliminated from the body by secretion into the bile and elimination in the stool.
Bile does not contain enzymes like other secretions of the gastrointestinal tract. Instead, it has bile salts (acids) that can:
- Emulsify fats and divide them into small particles. This is a bile-type detergent action.
- It helps the body absorb the decomposition products of fat in the intestine.
- Bile salts bind with lipids to form micelles. This is then absorbed through the intestinal mucosa.
- The other important function of bile is that it contains waste products from the breakdown of hemoglobin. This is called bilirubin and is normally made up of the body, since it eliminates old red blood cells that are rich in hemoglobin.
- Bile also carries excess cholesterol out of the body and “pours” it into the gastrointestinal tract where it can be distributed with other wastes.
Adult humans produce 400 to 800 ml of bile a day, and other animals produce proportionally similar amounts. The secretion of bile can be considered to occur in two stages:
- Initially, hepatocytes secrete bile into canaliculi, from which it flows to the bile ducts. This hepatic bile contains large amounts of bile acids, cholesterol and other organic molecules.
- As the bile flows through the bile ducts, it is modified by the addition of a bicarbonate-rich aqueous secretion from the ductal epithelial cells.
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 important route to eliminate cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it becomes soluble by bile acids and lipids such as lecithin.
Gallstones, most of which are predominantly cholesterol, are the result of processes that allow cholesterol to precipitate out of the solution in the bile.
Bile, either from the liver or gallbladder, contains the following substances:
- Bile salts.
- Fatty acids.
- Bicarbonate ions.
As mentioned, bile from the gallbladder is concentrated in comparison with hepatic bile. Bile salts are the largest volume of bile in the gallbladder and can be 6 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 cannalicules.
Bile acids are facial amphipathic, that is, they contain both hydrophobic (lipid soluble) and polar (hydrophilic) faces.
The cholesterol derived portion of a bile acid has a face that is hydrophobic (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 important functions:
Emulsification of lipid aggregates: bile acids have a detergent action on the fat particles of the diet that causes the fat globules to decompose or emulsify in tiny microscopic droplets.
Emulsification is not digestion per se, but it is important because it greatly 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 and 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.
In humans, approximately 500 mg of cholesterol 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.
Large amounts of bile acids are secreted in the intestine every day, but only relatively small amounts of the body are lost. This is because 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 pattern of recirculation; 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 flow of bile 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 and fats and partially digested proteins stimulate the secretion of cholecystokinin and secretin.
As discussed above, these enteric hormones have important 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 simulates the cells of the bile ducts to secrete bicarbonate and water, which expands the volume of bile and increases its flow to the intestine.