Bile

 Bile

Introduction- Bile is essential for life. Though it does not contain any digestive enzyme, it still is a very important digestive juice. Bile is both a product of secretion as well as of excretion of the liver. Minute droplets of bile collect inside the tiny vacuoles of the liver cells and are discharged into the bile capillaries through the intracellular canaliculi.

The primary bile capillaries start between hepatic cells as blind tubules. They join together repeatedly and form bigger channels and ultimately me out of liver as the right and left hepatic ducts. The two ducts unite and form the common bile duct, which enter into the duodenum, through the ampulla of Vater. Through the same ampulla also the pancreatic duct opens.

From the upper part of the common bile duct commences the cystic duct, which ends in the gall-bladder. Formation of bile by the liver is an active process. But entry of bile into the duodenum is intermittent and takes place only after meal.

Characteristics

Total quantity- 500-1,000 ml daily, on average about 700 ml.

Specific gravity- normally 1010-1011, Gall-bladder bile-1026-1040.

Color- It is yellowish green.

Taste- bitter,

Consistency-It is a viscid, mucoid liquid.

Reaction- liver bile alkaline -pH 7.7, gall bladder bile acidic-pH 5.6

Composition-

·        Water	89-98%
·        Solids	2-11%

 (1) Inorganic salts- chlorides, carbonates phosphates Na, and Ca and NaHCO₃ the total base is equivalent to about 170ml of  (N/10) NaOH per of liver bile ( 300ml% in gall-bladder bile).

(2) Bile salts - sodium taurocholate and sodium glycocholate, these are the most important constituents and synthesized by the liver (secretion).

(3) Bile pigments- bilirubin and biliverdin (excretion)

(4) Cholesterol, lecithin and traces of fatty acids, etc.

Functions of bile- Bile serves the following functions:

1. Digestion- It is essential for complete digestion of fats and to some extent proteins and carbohydrates. This action is due to bile salts which act in the following ways-

(a) By reducing surface tension- It converts fats globules into an emulsion, thus larger area becomes available for Lipase to act. Due to this the process of digestion is quickened.

(b) Activating action- The bile salts, by virtue of the cholic acid radicle, act as a specific activator for different lipases.

(c) Solvent action- Bile acts as a good solvent. It serves as a good medium for the interacting fats and different lipases.

2. Absorption- Due to presence of bile salts bile helps in the absorption of various substances. The following things are absorbed with the help of bile-

 (a) Fats- Bile is essential for fat absorption. This is carried out in two ways-

(i) Hydrotropic action-By this property the insoluble fatty acids, cholesterol, calcium, fat soluble vitamins, etc are made readily soluble in the watery contents of intestinal canal. In this way they are made easily diffusible and thus suitable for absorption. This action is brought about by the combination of these substances with bile acids. Fatty acids, cholesterol and many such insoluble substances make loose compounds with desoxycholic acid. Such compounds are soluble in water and are called choleic acids.

(ii) By reducing surface tension- Bile salts reduce the surface tension of the absorbing epithelium, increase their permeability and thus facilitate absorption.

(b) Iron, calcium and probably other mineral constituents of diet.

(c) Vitamins- Bile salts help in the absorption of lipid-soluble vitamins A, D, E,K and provitamin carotene.

III. Excretion- certain substances are excreted through bile, for instance,

(a) Some metals like copper, zinc, mercury, etc.

(b) Toxins

(c) bacteria, etc.

(d) Bile pigments

(e) Cholesterol and lecithin are probably chiefly excretory products.

IV. Laxative action- Bile salts stimulate peristalsis.

V. Cholagogue action- Bile acts as its own stimulant. Bile salts are the strongest cholagogues. They are absorbed from intestine, carried to liver and stimulate further bile secretion. The taurocholate is stronger in this respect than the glycocholate.

VI. Maintenance of a suitable pH- Bile helps to maintain a suitable pH of the duodenal contents and thus helps the action of all the enzymes. Bile is an important source of alkali for neutralizing the hydrochloric acid entering the intestine from stomach.

VII. Lecithin and cholesterol, present in bile, also help in the action of bile. First, they are reabsorbed with digested food. Secondly, they act as adjuvants to bile salts in the process of emulsification of fats (but on the whole they are regarded as excreted products).

VIII. Buffer and a lubricant- Mucin of bile acts as a buffer and a lubricant. IX. Regurgitation of bile in the stomach helps to neutralize gastric acidity and thus prevents the injurious effect of acids on gastric mucosa.