SECRETION OF BILE BY THE LIVER &FUNCTION OF BILE:

1. SECRETION OF BILE BY THE LIVER &FUNCTION OF BILE: One of many function of liver is to secrete bile , normally between 600 – 1200 ml/day , bile serve 2 important functions : 1st plays important role on fat digestion & absorption , because bile acid in bile do 2 things: 1- help to emulsify the large fat particles in to small particles that can be digested by lipase enzyme. 2- help in the transport& absorption of the digested fat end products to & through the intestinal mucosal membrane 2nd bile serve as a means for excretion of several waste products from the blood , these include bilirubin& excess of cholestrol, :

2. Storage &concentarion of bile in the gall bldder The bile secreted continually by the liver cell, normally it stored in the G.B. until nedded in the duodenum , the maximum volume of G.B.is 30 – 60 ml, nevertheless , as much as 12 hours of bile secretion (about 450ml)can be stored in the G.B.because water, Na+&Cl-& most other small electrolytes are continually absorbed by the G.B.mucosa concentrating the bile constituant including the bile salt , cholesterol , lecithin &bilirubin, bile is normally concentrated in this way 5 fold &maximum is 20 fold. Emptying of the G.B. The G.B.begins to empty, especially as fatty food enter the duodenum , about 30 minutes after a meal , the basic cause of emptying is rhythmical contraction of the wall of G.B., but effective emptying requires simultaneous relaxation of the sphincter of oddi that guide the exit of of the CBD to the duodenum . The most potent stimulus for G.B . Contraction is cholecystokinin (the same hormone which increase the secretion of pancreatic acini), its secretion stimulated by fatty food in the duodenum . The G.B.stimulated less strongly by acetylcholine secreting nerve fiber , from both vagi&enteric nervous system .

3. Even with strong contraction of G.B.emptying can be difficult because the sphincter of oddi normally remain tonically contracted , therefor before emptying of G.B. occure , the sphincter of oddi must be relaxed , 3 facters help in this : 1- cholecystokinin usually this effect is not suffecient alone to allow significant emptying , 2- rhythmical contraction of G.B transmit peristaltic wave down the CBD to the sphincter of oddi , causing relaxation of this sphincter , this too usually not enough to allow large amount of emptying . 3- when intestinal peristaltic wave travel over the wall of the duodenum itself , the the relasxation phase of waves relaxes the sphincter of oddi along with relaxation of the wall of the gut, this seems to be the most potent facter of all.

4. Bile salts &their function: Liver cell synthesize about 0.6gm of bile salt daily , the precursor of bile salt is cholesterol,which either supplied in the diet or synthesized in the liver cells during fat metabolism &then converted to cholic acid or chenodeoxy cholic acid , these acids then combined with glycine&to lesser extent with taurine to form glyco &tauro- conjugated bile acids, the salts of these acids secreted in the bile . The bile salts has 2 important functions : 1- they decrease the surface tension of fat particles in the food & allow to break the fat globule in to minute sizes this is called emulsifing or detergent function of bile salts. 2- bile salts help in the absorption of fatty actds,monoglycerids , cholesterole &other lipid from ihe intestinal tract they do this by forming minute complexes called micells they are hiohly soluble because of the electrical charge of the bile salts , the bile are ferried in this form to the mucosa where then they are absorbed , with out the presence of bile salts in the intestine up to 40%of the ingested lipid are lost in the stool&the person develop metabolic deficit because of nutrient loss,

5. Enterohepatic circulation of bile salts: About 94%of the bile salt are reabsorbed by the S.I , about one half of this by diffusion through the mucosa in the early portion of the S.I.&the remainder by an active transport process through the intestinal mucosa in the distal ileum , they inter the blood &pass to the liver . On reaching the liver , these salts are absorbed almost totally on the 1st passage through the venous sinusoids into the hepatic cells & then resecreted in to the bile , in this way about 94%of all bile salts are re circulated in to the bile , the small quantity of bile lost in the feces are replaced by new amount formed by the liver cells . This recirculatation of the bile salts called enterohepatic circulation . The quantity of bile secreted by the liver each day is highly dependant on the availability of bile salts , this mean that the daily rate of bile salt secretion is actively controlled by the availability of the bile salt in the enterohepatic circulation

6. Role of secretin in controlling bile secretion: The hormone secretin via blood stream stimulate secretion of bicarbonate – rich watery solution by the epithelial cells of bile ductules &ducts &not increased secretion by the liver cells themselves.the bicarbonate in turn pass in to the S.I.& join bicarbonate from the pancreas in neutralizing acid from stomach . Thus secretin feed back mechanism for neutralizing duodenal acid operate not only through its effect on pancreatic secretion but also through its effect on the secretion by the liver ductuls & ducts.

7. Secretion of cholesterole (gall stone formation): Bile salts formed in the hepatic cells from cholesterol &in the process of secreting the bile salts about 1 – 2 gm / day cholesterole is secreted into the bile . Cholesterol is completely insoluble in pure water , but the bile salts &lecithin in bile combine physically with the cholesterol to form micells that are soluble , when the bile become concentrated in the G.B., the bile salts &lecithen become concentrated along with cholesterol, which keep the cholesterol in solution .under abnormal condition cholesterol may precipitate result in forming cholesterol gall stones , due to : Too much absorption of water from the bile Too much absorption of bile salt & lecithin from bile Too much secretion of cholesterol Inflamation of the epithelium of G.B. Medical therapy for dissolving gall stones “ Simple cholesterol gall stones dissolved by chenodeoxycholic acid this can cause dissolution&reabsorption of gall stones by “ 1- the increased quantity of bile acids increases the volume of bile formed &therefore decrease the concentration of chlesterole in the bile 2- the increased quantity of bile acid in bile makes the cholesterolmore soluble 3- exogenous adminstration of bile acid decrease formation of bile acids by the liver which reduces the concentration of cholesterol.