Bile: Role, Properties and Functions | Digestion

In this article we will discuss about Bile:- 1. Role of Bile 2. Properties of Gall-Bladder Bile 3. Daily Formation 4. Composition 5. Bile Acids 6. Bile Salts 7. Clinical Significance 8. Bile Pigments 9. Enterohepatic Circulation 10. Functions.

Contents:

1. Role of Bile
2. Properties of Gall-Bladder Bile
3. Daily Formation of Bile
4. Composition of Bile
5. Bile Acids
6. Bile Salts
7. Clinical Significance of Bile
8. Bile Pigments
9. Enterohepatic Circulation of Bile
10. Functions of Bile

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1. Role of Bile:

a. Bile is absolutely necessary for the diges­tion of fat.

b. Liver plays an important role in digestion by producing bile. The gall-bladder, at­tached to the hepatic duct, stores a certain amount of bile produced by the liver be­tween meals. The composition of the bile in the gall-bladder is modified by addi­tion of mucin and other substances and by removal of water, bicarbonate and chlo­ride by reabsorption by the bladder mu­cosa.

During digestion, the gall-bladder contracts by the stimulation of the hor­mone cholecystokinin which is produced by the small intestine and release bile rap­idly to the small intestine by the way of common bile duct. The pancreatic secre­tions mix with the bile.

2. Properties of Gall-Bladder Bile:

a. Gall-bladder bile may be golden-yellow, brownish-yellow or olive-green in colour depending on the proportions of the bile pigments.

b. It is a viscid fluid.

c. It has a bitter taste and characteristic smell.

d. The inorganic material is mainly Na⁺, K⁺, Ca⁺⁺, CI, HCO₃⁻.

3. Daily Formation of Bile:

About 300 to 1,200 ml of bile formed daily in adult human beings.

4. Composition of Bile:

The composition of hepatic bile differs from that of gall-bladder bile which is shown in the fol­lowing table:

5. Bile Acids:

a. Bile acids are synthesized in the liver from cholesterol.

The synthesis of cholic acid from cholesterol is given:

Cholesterol → 7-hydroxy cholesterol → 3, 7-dihydroxy cholestane → 3, 7, 12-tri- hydroxy-cholestane → 3, 7, 12-trihydroxy cholestanoyl-CoA → cholyl-CoA → cholic acid.

b. The bile acids are derived from the parent acid called cholanic acid. The structure of cholic acid and cholanic acid are given (Figs. 16.3, 16.4).

c. The different bile acids are hydroxy de­rivatives of cholanic acid which are given below:

d. Many substances such as fatty acids, phenols, higher alcohols, camphor, naph­thalene, combine with deoxy-cholic acid in various molecular proportions. The re­sulting compounds are called choleic ac­ids. These choleic acids are water soluble. By such combinations insoluble fatty ac­ids, cholesterol, fat-soluble vitamins, drugs are rendered soluble and diffusible and thus capable of being absorbed.

6. Bile Salts:

a. Cholic acid + Glycine → Glycocholic acid.

b. Cholic acid + Taurine → Taurocholic acid.

Sodium or potassium glycocholate and sodium or potassium taurocholate are the two bile salts.

c. In human bile, sodium or potassium gly­cocholate predominates. It is three times as much as sodium or potassium taurocho­late.

Functions:

a. Bile salts act as emulsifying agents and emulsify fats increasing surface area and fats miscible with water. This helps to hydrolyse fats by pancreatic lipase.

b. They activate pancreatic lipase and cho­lesterol esterase.

c. They combine with free fatty acids and monoglycerides to form minute particles called miscelles and help in their absorp­tion in the intestines.

d. They stimulate intestinal peristalsis.

e. They stimulate bile production in the liver. They follow enterohepatic circulation and cause continuous secretion of bile by the liver.

f. Cholesterol remains soluble in gall-blad­der bile by bile salts.

g. They aid in absorption of fatty acids, cholesterol, carotene, and the fat-soluble vitamins D and K by forming complexes more soluble in water (hydrotropic action)’.

7. Clinical Significance of Bile:

a. Bile salts in the Wood are increased greatly in clinical obstructive jaundice.

b. After prolonged obstruction, the concentration of bile salts in the blood may diminish due to diminished synthesis of these substances as a result of progressive hepatocellular damage.

c. In the absence of bile salts, gall-stones are formed.

8. Bile Pigments:

a. Biliverdin and Bilirubin are the bile pig­ments formed from the breakdown of hemoglobin.

b. They are excreted in the bile.

c. They have no physiological role.

9. Enterohepatic Circulation of Bile:

A portion of the bile acids in the intestine un­dergoes changes by the activity of the intestinal bacteria. The de-conjugation and 7α-hydroxylation produce the secondary bile acids, deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid. The conjugated and unconjugated bile salts are absorbed almost in the ileum.

As fecal bile acids are present as the prod­ucts of bacterial metabolism, it is assumed that metabolism within the intestinal lumen with reabsorption by passive diffusion is a component of the enterohepatic circulation. This mechanism helps to return 90% of the bile acids secreted into the intestine to the liver each day. But lithocholic acid is not reabsorbed to any significant extent due to its insolubility.

500 mg of bile salts per day are not absorbed and is eliminated in the feces. The enterohepatic circulation of bile salts is so efficient that a small amount of bile acids is cycled through the intes­tine 6-10 times a day with the loss of a small amount in the feces.

10. Functions of Bile:

a. Bile salts help to lower the surface ten­sion of water and thus emulsify fats in the intestine and dissolve fatty acids and wa­ter-insoluble soaps. The presence of bile in the intestine helps the digestion and absorption of fats and the absorption of fat-soluble vitamins A, D, E and K.

b. Bile salts are activators of lipase.

c. Choleic acid formed by deoxycholic acid assists the absorption of many important insoluble compounds.

d. Bile salts are reabsorbed from the intes­tine and pass back to the liver where they stimulate further secretion of the bile (cholagogue action).

e. Bile is an important source of alkali which helps to neutralize the acid chyme from the stomach.

f. Bile is an important channel for the excre­tion of some substances like bile pigments, many drugs, toxins, and various inorganic substances such as copper, zinc and mer­cury.

g. Fat digestion is impaired in the absence of bile. The fat then covers the other food particles and prevents enzymes from at­tacking them. These undigested food par­ticles ultimately leads to putrefaction in the large intestine.

h. Bile salts keep cholesterol in solution in gall-bladder bile. In the absence of bile salts, cholesterol becomes precipitated. This results in the formation of gall-stones.