Bile is a digestive juice secreted from the liver, the largest gland of the body (mainly from the hepatocyte cells present in the liver). It is a watery golden yellow or greenish fluid mixture, containing bile pigments, bile salts, cholesterol and phospholipid etc. It is poured into the bile canaliculi from where it is ultimately goes to the common hepatic duct which joins with cystic duct to form common bile duct. Through cystic duct bile is directed to the gall bladder and stored and concentrated here. The rate of bile secretion is 800-1200ml/day. Bile plays different types of functions in our body but it’s main function is digestion and absorption of fat. The major constituent of bile is bile salt, which plays the main functions in digestion and absorption of fat, in the small intestine.
FORMATION OF BILE: The bile is formed by the hepatocytes and ductal cells in the lining of the hepatic duct. The one surface of the hepatocytes adjacent to the blood sinusoids and other to the bile canaliculi. The bile pigment is secreted into the bile canaliculi through osmotic secretion from the sinusoidal blood and the bile acids or bile salts are secreted from the hepatic cells and ultimately a mixture is secreted into the biliary canaliculi. After secretion bile is stored into the Gall bladder and from there it is released into the intestine when it required.
COMPOSITION OF BILE: Bile is a composition of 97.6% of water and 2.4% of solids like some organic and inorganic substances. Organic substance includes bile salt, bile pigment, cholesterol, fatty acids and phospholipids. And the inorganic substance includes some electrolytes like Sodium ion (Na+), Potassium ion (K+), Calcium ion (Ca++), Chlorin, etc.
a. BILE SALTS: Bile salt is a major constituent of bile. Bile salts are sodium and potassium salts of bile acids, which are derived from cholesterol. Primary bile acids are cholic acid and chenodeoxycholic acid which are synthesized by the hepatocyte cells present in the liver. Secondary bile acids are deoxycholic acid and lithocholic acid. In liver the bile acids are conjugated with either glycine or taurine, forming the conjugated bile acids. Then from there they enter into the intestine. Bile salts helps in fat digestion by emulsifying fat drops and also help in the absorption of fat by micelle formation.
b. BILE PIGMENTS: The two principal bile pigment- Bilirubin and Biliverdin are the other major constituent of bile. These pigments are formed from the. These pigments are metabolites of haemoglobin, formed during the breakdown of haemoglobin (Hb, which are released from the RBC) in the liver. The hepatic cells present in the liver, extract those pigments from the blood and conjugate them with the glucuronic acid. These are responsible for the golden yellow colour of bile. Bile pigments have no digestive function.
c. CHOLESTEROL: Cholesterol is another important constituent of bile. It does no digestive function. It seems to be the by-product of bile salt synthesis in the hepatocyte cells in the liver.
d. PHOSPHOLIPIDS: Phospholipids are the most abundant organic compound of bile just after the bile salts. The phospholipid includes primarily lecithin.
e. ELECTROLYTES: The inorganic substance includes some electrolytes mainly sodium, potassium, calcium, chlorin.
FUNCTION OF BILE: Bile served its function in the duodenum, and its main functional constituent is bile salts. Bile plays many different types of functions of our body.
- Digestive Function: bile salts help in the digestion of the fats by emulsification of the fat drop. Sodium glycocholate and Sodium taurocholate break the large fat droplets into smaller and easily digestible one.
- Absorptive Function: It also helps in the absorption of fat by micelle formation and fat-soluble vitamins (like vitamin A, D, E and K) in small intestine.
- Excretory Function: Bile pigments (bilirubin and biliverdin) are the major excretory products of the bile. Some heavy metal (like copper and iron), some toxins, cholesterol, some bacteria (like typhoid bacteria) etc are also excreted in bile.
- Prevention of the Formation of Gall Stone: Bile salt prevent the stone formation in the gall bladder by keeping cholesterol and lecithin in the solution.
- pH Maintenance: Bile also helps to maintain the pH of the Gastrointestinal tract. Bile is highly alkaline so, bile juice neutralizes the gastric HCL, which is present in the chyme entering the small intestine.
- Protective Function: Bile inhibit the growth of certain bacteria present in the lumen of the small intestine. For this reason, bile is also known as natural detergent.
Bile plays many other functions like- it helps to lubricate the wall of the small intestine, bile salts increase the gastrointestinal motility and acts as laxative, it also helps to activate the enzyme lipase.