The basic functional unit of the liver is the liver lobule, which is constructed around a central vein.The lobule is composed of many hepatic cellular plates that radiate away from the central vein. Between the cells in the hepatic plates is the bile canaliculi that collect the bile secreted by the liver cells and empty them into terminal bile ducts. These bile ducts join with the cystic duct to form the common bile duct in the species which have gall bladder.
The common bile duct terminate as a slight swelling in the duodenal mucosa as bile duct papilla which is controlled by a sphincter muscle known as Sphincter of oddi.This sphincter is well defined in carnivores but not so prominent in herbivores.
In human, dogs, cats and horses the bile and pancreatic ducts open closely together into the duodenum. In sheep and goats the pancreatic duct empties directly into the common bile duct so that the mixture of bile and pancreatic juice enters the duodenum. In pigs and cows bile and pancreatic ducts lie some distance apart.
Bile is secreted continuously in all species.Bile is stored in the gall bladder in animals which feed once or twice daily. Gall bladder is the storage organ of bile and has a ability to concentrate the bile up to 20-30 times during inter digestive periods by the absorption of NaCl or NaHCO3 and H2O . So gall bladder bile differs in composition as that of hepatic bile. The degree of concentrating ability varies with species of animals. In ruminants and pigs only slight absorption of water takes place. More over these animals are continuous feeders hence need continuous flow of dilute bile in to the intestine. In horse, gall bladder is absent, hence large and continuous flow of hepatic bile to the duodenum.
Emptying of the gall bladder
Basic conditions are necessary for the gall bladder to empty.
Relaxation of sphincter of oddi allows the bile to flow from the common bile duct into the duodenum
Contraction of the gall bladder provides the force required to move the bile along the common duct. Meal rich in fat, shows both of these effects.
Fat as well as partially digested proteins in the food entering into the small intestine causes the release of cholecystokinin from the intestinal mucosa of upper small intestine. It is absorbed through circulation passes to gall bladder, causes contraction of the gall bladder muscle. This provides the pressure that forces bile towards the duodenum.
Vagal stimulation associated with the cephalic phase of gastric secretion causes an additional weak contraction of the gall bladder.
Neurogenic or a myogenic reflex from the gall bladder to the sphincter of oddi inhibits sphincter of oddi. Chloecytokinin may also have a direct inhibitory action on the sphincter.
Presence of food in the duodenum causes increased peristalsis in the duodenal wall. When peristaltic wave travels towards the sphincter of oddi momentary relaxation referred as “Receptive Relaxation” of sphincter of oddi occurs before the peristaltic contraction of sphincter of oddi adjacent to intestinal wall.
Formation of bile
Bile secretion involves two components.
First is the bile salt dependant secretary flow.
Bile is a major route of cholesterol excretion. Bile salts are synthesized in the liver from cholesterol, have cyclo pentano per hydro phenanthrene ring structure.
The primary bile acids in most species are cholic acid and chenodeoxy cholic acid conjugated with either taurine or glycine which lower the pH of bile salt .
The secondary bile acids (deoxy cholic acid and litho colic acid) are derived from bacterial de hydroxylation in the colon.
the pH of intestinal content is close to pH 6, leads to failure of conjugation and accumulation of large amounts of un-ionized, hydrogenated bile acids.
Limited H2O solubility of these acids leads to absorbtion by a passive diffusion. Thus conjugation of bile salt provides concentration of ionized bile salts which is required for normal fat absorption.
The bile salts are actively transported from the hepatocyte into the bile canaliculi. Bile is iso osmotic because of passive flow of cations and H2O. Bile salts are re circulated from the intestine(Entero hepatic circulation) which inturn stimulates hepatic bile flow.
Second component of the bile secretion is the bile salt independent flow and involve the ductular epithelium. Active transport of Na+ accompanied by Cl, HCO3, and H2O provides buffer to the duodenum. This phase is under the control of secretin, cholecystokinin, and gastrin to increase HCO3 concentration and bile flow.
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