PROPERTIES AND COMPOSITION OF BILE

PROPERTIES AND COMPOSITION OF BILE

PROPERTIES OF BILE

Volume : 800 to 1,200 mL/day

Reaction : Alkaline

pH : 8 to 8.6

Specific gravity : 1.010 to 1.011

Color : Golden yellow or green.

COMPOSITION OF BILE

Bile contains 97.6% of water and 2.4% of solids. Solids include organic and inorganic substances.

SECRETION OF BILE

Bile is secreted by hepatocytes. The initial bile secreted by hepatocytes contains large quantity of bile acids, bile pigments, cholesterol, lecithin and fatty acids. From hepatocytes, bile is released into canaliculi. From here, it passes through small ducts and hepatic ducts and reaches the common hepatic duct. From common hepatic duct, bile is diverted either directly into the intestine or into the gallbladder.

Sodium, bicarbonate and water are added to bile when it passes through the ducts. These substances

are secreted by the epithelial cells of the ducts. Addition of sodium, bicarbonate and water increases the total quantity of bile.

STORAGE OF BILE

Most of the bile from liver enters the gallbladder, where it is stored. It is released from gallbladder into the intestine whenever it is required. When bile is stored in gallbladder it undergoes many changes both in quality and quantity such as:

1. Volume is decreased because of absorption of a large amount of water and electrolytes (except

calcium and potassium)

2. Concentration of bile salts, bile pigments, cholesterol, fatty acids and lecithin is increased because of

absorption of water and electrolytes

3. The pH is decreased slightly

4. Specific gravity is increased

5. Mucin is added to bile.

BILE SALTS

Bile salts are the sodium and potassium salts of bile acids, which are conjugated with glycine or taurine.

FORMATION OF BILE SALTS

Bile salts are formed from bile acids. There are two primary bile acids in human, namely cholic acid and

chenodeoxycholic acid, which are formed in liver and enter the intestine through bile. Due to the bacterial action in the intestine, the primary bile acids are converted into secondary bile acids:

Cholic acid deoxycholic acid

Chenodeoxycholic acid lithocholic acid

Secondary bile acids from intestine are transported back to liver through enterohepatic circulation. In liver,

the secondary bile acids are conjugated with glycine (amino acid) or taurin (derivative of an amino acid) and form conjugated bile acids, namely glycocholic acid and taurocholic acids. These bile acids combine with sodium or potassium ions to form the salts, sodium or potassium glycocholate and sodium or potassium taurocholate .

ENTEROHEPATIC CIRCULATION OF BILE SALTS

Enterohepatic circulation is the transport of substances from small intestine to liver through portal vein. About 90% to 95% of bile salts from intestine are transported to liver through enterohepatic circulation. Remaining 5% to 10% of the bile salts enter large intestine. Here, the bile salts are converted into deoxycholate and lithocholate, which are excreted in feces.

FUNCTIONS OF BILE SALTS

Bile salts are required for digestion and absorption of fats in the intestine. The functions of bile salts are:

1. Emulsification of Fats

Emulsification is the process by which the fat globules are broken down into minute droplets and made in the form of a milky fluid called emulsion in small intestine, by the action of bile salts.

Lipolytic enzymes of GI tract cannot digest the fats directly because the fats are insoluble in water due to the surface tension. Bile salts emulsify the fats by reducing the surface tension due to their detergent action. Now the fats can be easily digested by lipolytic enzymes. Unemulsified fat usually passes through the intestine and then it is eliminated in feces.

Emulsification of fats by bile salts needs the presence of lecithin from bile.

2. Absorption of Fats

Bile salts help in the absorption of digested fats from intestine into blood. Bile salts combine with fats and

make complexes of fats called micelles. The fats in the form of micelles can be absorbed easily.

3. Choleretic Action

Bile salts stimulate the secretion of bile from liver. This action is called choleretic action.

4. Cholagogue Action

Cholagogue is an agent which causes contraction of gallbladder and release of bile into the intestine. Bile

salts act as cholagogues indirectly by stimulating the secretion of hormone cholecystokinin. This hormone causes contraction of gallbladder, resulting in release of bile.

5. Laxative Action

Laxative is an agent which induces defecation. Bile salts act as laxatives by stimulating peristaltic movements of the intestine.

6. Prevention of Gallstone Formation

Bile salts prevent the formation of gallstone by keeping the cholesterol and lecithin in solution. In the absence of bile salts, cholesterol precipitates along with lecithin and forms gallstone.

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