gallbladder & bile physiological aspects

Gallbladder & Bile

Dr Raghuveer Choudhary

Saturday, April 21, 2012

When no chyme is in the small intestine, the hepatopancreatic sphincter is closed. Bile flows into gallbladder.

cystic duct

common hepatic duct

gallbladder

common bile duct

hepatopancreatic sphincter

pancreas

duodenum

Function of Gallbladder - absorbs water and stores the bile for later use.

gallbladder

hepatopancreatic sphincter

duodenum

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Liver and Gall Bladder

The liver has many functions, one of which is to produce bile to aid in the digestive process.

Bile acts to help neutralize the chyme as it enters the small intestine and is a fat emulsifier.

It breaks up fats into tiny particles so that they are more accessible to digestive enzymes.

The gallbladder is chiefly a storage organ for bile.

Gallbladder & Bile

• Sac on underside of liver• Stores and concentrates bile• Bile – yellow – green fluid• Contains minerals, cholesterol, neutral fats,

phospholipids, bile pigments, bile acids (salts)• Bile salts aid in digestion of fats• Excessively concentrated = gallstones• 80% bile salts recycled• 20% bile – excreted in feces

• Sac-like organ attached to the inferior surface of the liver.

• Stores and concentrates bile.• When gallbladder fills with bile, it expands.

– Contraction of the muscularis layer of the gallbladder, ejects bile into the common bile duct into duodenum.

• When small intestine is empty, sphincter of Oddi closes.– Bile is forced up to the cystic duct to gallbladder.

Gallbladder

The Gallbladder and Bile Modification

• Full gallbladder contains 40–70 ml bile• Bile composition gradually changes in

gallbladder:– water is absorbed– bile salts and solutes become concentrated

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Regulation of bile release

The liver makes bile continuously.When there is no food in the small

intestine, the hepatopancreatic sphincter (the entrance of the common bile duct and pancreatic duct into the small intestine) is closed and the bile backs up into the gallbladder.

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Regulation of bile release

When food enters the small intestine, activation of mechano and chemoreceptors leads to parasympathetic stimulation.

This mildly stimulates gallbladder contraction

This also stimulates the release of cholecystokinin and secretin from the duodenal and enteroendocrine cells.

Bile Release

- is constantly produced by the liver (500-1,000 mL of bile per day).

- is a yellow-green fluid containing :

minerals

bile pigments

bile acids

phospholipids

cholesterol

neutral fats

Bile

facilitate fat digestion and absorption

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Composition of bile

Bile is an alkaline solution containing a number of components.

The important ones for digestion are bile salts, cholesterol and phospholipids.

Bile salts act as a fat emulsifier – they break globules of fat entering the small intestine into millions of fatty droplets.

This provides a large surface area for the fat digesting enzymes to work on.

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Composition of Bile

Bile Production and Secretion (continued)

• Bile acids are derivatives of cholesterol.– Major pathway of

cholesterol breakdown in the body.

• Principal bile acids are:– Cholic acid.– Chenodeoxycholic acid.

• Combine with glycine or taurine to form bile salts.

– Bile salts aggregate as micelles.

• 95% of bile acids are absorbed by ileum.

Insert fig. 18.25

Biliary SecretionA. Bile is secreted continuously by the liver, and the rate of secretion depends on

whether a fed or fasting state exists.

B. Bile contains bile salts, lecithin (a phospholipid), cholesterol, bile pigments (eg, bilirubin), water, and electrolytes.

C. Bile constituents are dissolved in an alkaline solution resembling pancreaticjuice. Bile plays an important role in the intestinal digestion and absorption oflipids.

D. Primary bile acids—cholic acid and chenodeoxycholic acid—are synthesizedby the liver from cholesterol. The lipid-soluble bile acids are conjugated with either glycine or taurine.

Because they are ionized at neutral pH, conjugated bile acids exist as salts of sodium or potassium and, therefore, are known as bile salts.

F. Secondary bile acids are formed by deconjugation and dehydroxylation of the primary bile salts by intestinal bacteria, forming deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid.

G. Lithocholic acid is hepatotoxic and is normally excreted in feces.H. The bile acid pool, which under normal conditions is constant in size (about 2–4 g), is a mass of primary and secondary bile acids.

1) Fats are first broken up into smaller emulsification droplets by lecithin and bile salts (acids) in the bile.

2) When lipase digests fats, the products are two fatty acids (FFAs) and a monoglyceride.

3) Bile salts coat these and other lipids and form droplets called micelles.

Micelles release their lipids, which diffuse freely across the plasma membrane.

Resynthesis of triglycerides. Coating with protein, forming droplets called chylomicrons.

Intestinal absorptive cell

Chylomicrons are too large to enter blood capillaries and must be first transported in the lymphatic lacteal.

Fat globule

emulsification droplets

micelles

chylomicrons

Intestinal lumen Intestinalepithelialcells

lacteal blood

Bilesalts

lipase &bile salts

Effects of CholecystectomyThe periodic discharge of bile from the gallbladder aids digestion but is not essential for it. Cholecystectomized patients maintain good health and nutrition with a constant slow discharge of bile into the duodenum, although eventually the bile duct becomes somewhat dilated, and more bile tends to enter the duodenum after meals than at other times. Cholecystectomized patients can even tolerate fried foods, although they generally must avoid foods that are particularly high in fat content.