Lec.8 GIT 19 Apr 2016
Biliary Tree (Biliary Tract)
Objectives
Physiological anatomy of biliary tree
Liver: functions
Secretion of bile
Physiological anatomy of biliary secretion
Composition of bile
Gallbladder
Control of biliary secretion
Physiological anatomy of biliary tree
The other accessory organs of GIT are biliary tree or biliary tract which includes (liver, gallbladder and bile ducts. Bile ducts includes (right and left hepatic ducts , common hepatic duct which joins with cystic duct from gallbladder forming common bile duct, common bile duct joins with pancreatic duct which enters duodenum through sphincter of Oddi) .
Liver
Why liver is considered as an important organ in our body?
Liver is the largest organ in the body located in the upper right and central portions of the abdominal cavity, just below the diaphragm. The main Functions of the liver are: 1. Formation &secretion of bile .2.vascular functions for filtration and storage of the blood.3.metabolic functions concerned with metabolism of carbohydrate, proteins, hormones and foreign chemicals. 4.storage of fat soluble vitamins( A,D,E,&K) & iron. 5. Synthesis of plasma proteins. 6. Produce clotting factors.7.detoxify, a process of removing toxins and many drugs from the blood.
8. Formation of heparin, a substance that reduces the viscosity of blood by limiting its ability to coagulate.9.immunity functions: presence of Kuppffer's cells.
Formation and Secretion of Bile: Physiologic Anatomy of Biliary Secretion
Bile is secreted in two stages by the liver: (1) the initial portion is secreted by the principal functional cells of the liver, the hepatocytes; this initial secretion contains
large amounts of bile acids, cholesterol, and other organic constituents. It is secreted into minute bile canaliculi that originate between the hepatic cells.
2) The bile flows in the canaliculi toward the interlobular septa, where the canaliculi empty into terminal bile ducts and then into progressively larger ducts, finally reaching the hepatic duct and common bile duct. From these the bile either empties directly into the duodenum or is diverted for minutes up to several hours through the cystic duct into the gallbladder. In its course through the bile ducts, a second portion of liver secretion is added to the initial bile. This additional secretion is a watery solution of sodium and bicarbonate ions secreted by secretory epithelial cells that line the ductules and ducts.
Composition of bile
Normally about 600-1200 ml. of bile is secreted per day by the liver. The main constituents of bile are:
Bile salts most abundant substance secreted in the bile , accounting for about one half of the total solutes of bile.
Bile pigments eig. Biliverdin and bilirulin.
Lipid substance eig. cholesterol , lecithin, and fatty acids
water
Electrolytes such as. HCO3 - , Na + , CL - , Ca ++ , K +
Alkaline phosphates.
Gallbladder
A saclike structure about 10 cm. long that located behind the liver. ItsConnected to common hepatic duct by cystic duct. The main functions of gallbladder are 1.to store and concentrate the bile. Bile is secreted continually by the liver cells, but most of it is normally stored in the gallbladder until needed in the duodenum. The maximum volume that the gallbladder can hold is only 30 to 60 milliliters. When the gallbladder is empty or has a small amounts of bile ,the gallbladder mucosa has honeycomb-like folds that are ready to expand as bile enters. Nevertheless, as much as 12 hours of bile secretion (usually about 450milliliters) can be stored in the gallbladder because water ,Na, K ,Cl , and HCO3- are continually absorbed through the gallbladder mucosa, concentrating the remaining bile constituents that such as bile salts, cholesterol, lecithin, fatty acids ,Ca ions and bilirubin.
2. Acidification of the bile is another function of the gallbladder PH of bile in liver is 7.8 8.6 mainly alkaline whereas pH of bile in gallbladder is acidic 7.0 7.4 due to reabsorption of HCO3- by gallbladder mucosa.
Emptying of the Gallbladder: Substances that cause contraction of the gallbladder are called cholagogues 1.hormonal: Stimulatory Role of CCK:.
the gallbladder empties its store of concentrated bile into the duodenum mainly in response to the CCK stimulus that itself is initiated mainly by fatty foods. When fat is not in the food, the gallbladder empties poorly, but when significant quantities of fat are present, the gallbladder normally empties completely in about 1 hour. The mechanism of gallbladder emptying is rhythmical contractions of the wall of the gallbladder, but effective emptying also requires simultaneous relaxation of the sphincter of Oddi, which guards the exit of the common bile duct into the duodenum. The stimulus for CCK entry into the blood from the duodenal mucosa is mainly the presence of fatty foods in the duodenum.2.Neural in addition to CCK the gallbladder is stimulated less strongly by acetylcholine-secreting nerve fibers from both the vagi and the intestinal enteric nervous system..
Bile Salts
The liver cells synthesize about 6 grams of bile salts daily. The bile salts are sodium and potassium salts of bile acids. The precursor of the bile salts is cholesterol, which is either present in the diet or synthesized in the liver cells during the course of fat metabolism. In the liver the cholesterol is first converted to two principal primary bile acids (cholic acid or chenodeoxycholic acid) in about equal quantities. These acids are conjugated to glycine or taurine, a derivative of cysteine to form glyco- and tauro-conjugated bile acids. The salts of these acids, mainly sodium salts, are then secreted in the bile.
in the liver Cholesterol --primary acids
Functions of bile salts: in the intestinal tract: First, bile plays an important role the digestion and absorption of fat .a. have detergent action of bile salts on the fat particles in the food. This decreases the surface tension of the particles and allows agitation in the intestinal tract to break the fat globules into minute sizes..b, bile salts help in the absorption of, fatty acids, cholesterol monoglycerides, and other lipids from the intestinal tract. They do this by forming very small physical complexes with these lipids; the complexes are called micelles they are semi-soluble in the chyme. Without the presence of bile salts in the intestinal tract up to 40 per cent of the ingested fats are lost into the feces, and the person often develops a metabolic deficit because of this nutrient loss. Condition is called (steatorrhea ). Stool is fatty, murky and clay colored stool because of the impaired digestion and absorption of fat. Second: help absorption of fat soluble vitamins such as A, D, E&K .So without bile salts there is severe malabsorption of fat soluble vitamins (A, D, E, and K). Vitamins (A, D, E) are stored in the body , this is not true for vitamins K .Within a few days the person develops a deficient formation of blood clotting factors by the liver such as prothrombin and factors V11,1X and X . Thus resulting in serious in serious impairment of blood coagulation.
Bile serves as a means for excretion of several important waste products from the blood. These include especially bilirubin, an end product of hemoglobin destruction, and excesses of cholesterol.
Enterohepatic circulation of bile salts
Recirculation of bile salts is called the enterohepatic circulation of bile salts.
If any substance which is secreted in the bile and which is reabsorbed from the G.I.T and return to the liver and to appear once again in the bile may be said to have undergone enterohepatic circulation .About 95% of the bile salts are reabsorbed by the small intestine, about one half of this in the early portions of the small intestine and the remainder to in the distal ileum .They enter the portal blood and pass to the liver. On reaching the liver these bile salts are resecreted into the bile, small amounts of bile salts about 5% enter the colon. In the colon, bacteria convert primary bile acids into secondary bile acids. About 1% of secondary acids is absorbed and enters portal blood and pass to the liver and resecreted into the bile. About 4% of bile salts lost into the feces and are replaced by new amounts formed continually by the liver cells.).
1. in the liver Cholesterol --primary acids
2. in colon by action of bacteria primary acids------secondary acids
. In addition to bile salts, there are other endogenous substances that undergo enterohepatic circulation for example: vitamin B12, folic acid, phospholipids, cholesterol and exogenous drugs such as digitalis.
Bile Pigments
it accounts for approximately 2% of the solids. which are excreted in the bile and then eliminated in the feces. One of these is the greenish yellow pigment bilirubin .This is a major end product of hemoglobin degradation. Bilirubin is insoluble in water. Bilirubin is highly soluble in all cell membranes and is also very toxic. Therefore, its excretion in the bile is one of the very important functions of the liver. Bilirubin and biliverdin are responsible for the golden yellow color of bile..Disorder metabolism of bile pigment is called jaundice (icterus).
Cholesterol
Bile is the primary excretory pathway for cholesterol. . Cholesterol is secreted by the hepatocytes into the bile.. about 1- 2 gm / day cholesterol is secreted into the bile .Cholesterol is insoluble in the water but physically combine with bile salts and lecithin in bile to form micelles that are soluble in the form of a colloidal solution, .When the bile becomes concentrated in the gallbladder, the bile salts and lecithin become concentrated along with the cholesterol, which keeps the cholesterol in solution. Under abnormal conditions. the cholesterol may precipitate in the gallbladder, resulting in the formation of cholesterol gallstone. The amount of cholesterol in the bile is determined partly by the quantity of fat that the person eats, because liver cells synthesize cholesterol as one of the products of fat metabolism in the body. For this reason, people on a high-fat diet over a period of years are prone to the development of gallstones.Control of Biliary Secretion
Any substance that increase the secretion of bile are known as choleretics..The flow of bile is increased by 1. Hormone secretin produced by the duodenum in response to acid in duodenum which carried through the circulation to the liver, stimulates bile secretion by the liver. 2.CCK produced by duodenum in response to fat and carried to gallbladder through circulation ,stimulates the gallbladder to contract, and sphincter of Oddi to relax, releasing bile into the duodenum.3.vagal nerve stimulation of the liver to secrete bile and gallbladder to release bile into the duodenum.