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Bile is essential for life. Although it does not contain any enzyme, yet, it acts as a very important digestive juice. Its importance is so much that, life cannot be maintained without it. If a cannula is inserted in the common bile duct and all bile is collected outside, it is seen that the dog develops various abnormalities of bone, anaemia, lack of nutrition and eventually dies (Whipple).
Bile serves the following functions:
1. Digestion:
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Bile is essential for the complete digestion of fats and to some extent of proteins and carbohydrates.
This action is due to the presence of bile salts, which act in the following ways:
a. By reducing surface tension, so that fats are converted into an emulsion. The fine globules of fat, due to their innumerable number, render a larger surface area for the enzyme (lipase) to act. Due to this the process of digestion is quickened.
b. Activating Action:
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The bile salts, by virtue of the cholic acid radicle, act as a specific activator for different lipases. [That this action is not due to emulsification is proved by the fact that, although emulsification is unnecessary for the digestion of water-soluble triacetin by pancreatic lipase, yet the action of the enzyme is accelerated by bile salts.]
c. Solvent Action:
Bile acts as a good solvent. Due to this property, it serves as a good medium for the interacting fats and fat-splitting enzymes.
2. Absorption:
Bile helps in the absorption of various substances. This is also due to presence of bile salts.
The following things are absorbed with the help of bile:
i. Fats:
Bile is essential for fat absorption.
This is carried out in two ways:
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(1) Hydrotropic Action:
By this property the insoluble fatty acids, cholesterol, calcium, soaps, 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 cholic acids.].
(2) Bile Salts:
Bile salts reduce the surface tension of the absorbing epithelium, increase their permeability and thus facilitate absorption.
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a. Iron, Calcium:
Iron, calcium and probably other mineral constituents of diet.
b. Vitamins:
Bile salts help in the absorption of lipid-soluble vitamins A, D, E and K; and pro-vitamin carotene.
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3. Excretion:
Certain substances are excreted through bile, for instance:
i. Some metals like copper, zinc, mercury, etc.
ii. Toxins, bacteria, etc.
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iii. Bile pigments. [A portion of these pigments is then excreted in the faeces and in urine in various forms.]
iv. Cholesterol and lecithin are probably chiefly excretory products.
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4. Laxative Action:
Bile salts stimulate peristalsis. When introduced directly into the colon it stimulates peristalsis of these parts.
5. Cholagogue Action:
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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.
6. Bile Helps to Maintain 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 neutralising the hydrochloric acid entering the intestine from stomach.
7. Lecithin and Cholesterol:
Lecithin and cholesterol, present in bile, also help in some ways:
First, they are treated as food and are reabsorbed.
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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).
8. Mucin of Bile:
Mucin of bile acts as a buffer and a lubricant.
9. Regurgitation of Bile:
Regurgitation of bile in the stomach helps to neutralise gastric acidity and thus prevents the injurious effect of acids on gastric mucosa.
From the above it will be evident that bile is important not only as a digestive juice but for also various other purposes.
