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Secretion of Bile and the Role of Bile Acids In Digestion

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/liver/bile.

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Bile is a complex fluid containing water, electrolytes and a battery

of organic molecules including bile acids, cholesterol,

phospholipids and bilirubin that flows through the biliary tract

into the small intestine. There are two fundamentally important

functions of bile in all species:

Bile contains bile acids, which are critical for digestion and

absorption of fats and fat-soluble vitamins in the small intestine.

Many waste products are eliminated from the body by secretion into

bile and elimination in feces.

Adult humans produce 400 to 800 ml of bile daily, and other animals

proportionately similar amounts. The secretion of bile can be

considered to occur in two stages:

Initially, hepatocytes secrete bile into canaliculi, from which it

flows into bile ducts. This hepatic bile contains large quantities

of bile acids, cholesterol and other organic molecules.

As bile flows through the bile ducts it is modified by addition of a

watery, bicarbonate-rich secretion from ductal epithelial cells.

In species with a gallbladder (man and most domestic animals except

horses and rats), further modification of bile occurs in that organ.

The gall bladder stores and concentrates bile during the fasting

state. Typically, bile is concentrated five-fold in the gall bladder

by absorption of water and small electrolytes - virtually all of the

the organic molecules are retained.

Secretion into bile is a major route for eliminating cholesterol.

Free cholesterol is virtually insoluble in aqueous solutions, but in

bile, it is made soluble by bile acids and lipids like lethicin.

Gallstones, most of which are composed predominantly of cholesterol,

result from processes that allow cholesterol to precipitate from

solution in bile.

Role of Bile Acids in Fat Digestion and Absorption

Bile acids are derivatives of cholesterol synthesized in the

hepatocyte. Cholesterol, ingested as part of the diet or derived

from hepatic synthesis is converted into the bile acids cholic and

chenodeoxycholic acids, which are then conjugated to an amino acid

(glycine or taurine) to yield the conjugated form that is actively

secreted into cannaliculi.

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 one 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.

Their amphipathic nature enables bile acids to carry out two

important functions:

Emulsification of lipid aggregates: Bile acids have detergent action

on particles of dietary fat which causes fat globules to break down

or be emulsified into minute, microscopic droplets. Emulsification

is not digestion per se, but is of importance because it greatly

increases the surface area of fat, making it available for digestion

by lipases, which cannot access the inside of lipid droplets.

Solubilization and transport of lipids in an aqueous environment:

Bile acids are lipid carriers and are able to solubilize many lipids

by forming micelles - aggregates of lipids such as fatty acids,

cholesterol and monoglycerides - that remain suspended in water.

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

soluble vitamins.

Role of Bile Acids in Cholesterol Homeostasis

Hepatic synthesis of bile acids accounts for the majority of

cholesterol breakdown in the body. In humans, roughly 500 mg of

cholesterol are converted to bile acids and eliminated in bile every

day. This route for elimination of excess cholesterol is probably

important in all animals, but particularly in situations of massive

cholesterol ingestion.

Interestingly, it has recently been demonstrated that bile acids

participate in cholesterol metabolism by functioning as hormones

that alter the transcription of the rate-limiting enzyme in

cholesterol biosynthesis.

Enterohepatic Recirculation

Large amounts of bile acids are secreted into the intestine every

day, but only relatively small quantities are lost from the body.

This is because approximately 95% of the bile acids delivered to the

duodenum are absorbed back into blood within the ileum.

Venous blood from the ileum goes straight into the portal vein, and

hence through the sinusoids of the liver. Hepatocytes extract bile

acids very efficiently from sinusoidal blood, and little escapes the

healthy liver into systemic circulation. Bile acids are then

transported across the hepatocytes to be resecreted into 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 instance, sick hepatocytes have

decreased ability to extract bile acids from portal blood and damage

to the canalicular system can result in escape of bile acids into

the systemic circulation. Assay of systemic levels of bile acids is

used clinically as a sensitive indicator of hepatic disease.

Pattern and Control of Bile Secretion

The flow of bile is lowest during fasting, and a majority of that is

diverted into the gallbladder for concentration. When chyme from an

ingested meal enters the small intestine, acid and partially

digested fats and proteins stimulate secretion of cholecystokinin

and secretin. As discussed previously, these enteric hormones have

important effects on pancreatic exocrine secretion. They are both

also important for secretion and flow of bile:

Cholecystokinin: The name of this hormone describes its effect on

the biliary system - cholecysto = gallbladder and kinin = movement.

The most potent stimulus for release of cholecystokinin is the

presence of fat in the duodenum. Once released, it stimulates

contractions of the gallbladder and common bile duct, resulting in

delivery of bile into the gut.

Secretin: This hormone is secreted in response to acid in the

duodenum. Its effect on the biliary system is very similar to what

was seen in the pancreas - it simulates biliary duct cells to

secrete bicarbonate and water, which expands the volume of bile and

increases its flow out into the intestine.

The processes of gallbladder filling and emptying described here can

be visualized using an imaging technique called scintography. This

procedure is utilized as a diagnostic aid in certain types of

hepatobiliary disease.

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The Liver: Introduction and Index

Physiology of the Hepatic Vascular System Biliary Excretion of

Waste Products: Elimination of Bilirubin

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Last updated on November 23, 2001

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