AO-MEDICINE

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) Next, 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. This second secretion sometimes increases the total quantity of bile by as much as an additional 100 per cent. The second secretion is stimulated especially by secretin, which
causes release of additional quantities of bicarbonate ions to supplement the bicarbonate ions in pancreatic secretion
(for neutralizing acid that empties into the duodenum from the stomach).

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. Nevertheless, as
much as 12 hours of bile secretion (usually about 450 milliliters) can be stored in the gallbladder because
water, sodium, chloride, and most other small electrolytes are continually absorbed through the gallbladder
mucosa, concentrating the remaining bile constituents that contain the bile salts, cholesterol,
lecithin, and bilirubin.
Most of this gallbladder absorption is caused by active transport of sodium through the gallbladder
epithelium, and this is followed by secondary absorption of chloride ions, water, and most other diffusible
constituents. Bile is normally concentrated in this way about 5-fold, but it can be concentrated up to a
maximum of 20-fold.



Emptying of the Gallbladder—Stimulatory Role of Cholecystokinin.

When food begins to be digested in the upper gastrointestinal tract, the gallbladder begins to empty,
especially when fatty foods reach the duodenum about 30 minutes after a meal. 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.
By far the most potent stimulus for causing the gallbladder contractions is the hormone cholecystokinin

This is the same cholecystokinin discussed earlier that causes increased secretion of digestive enzymes by the
acinar cells of the pancreas.The stimulus for cholecystokinin entry into the blood from the duodenal
mucosa is mainly the presence of fatty foods in the
duodenum.
In addition to cholecystokinin, the gallbladder is stimulated less strongly by acetylcholine-secreting
nerve fibers from both the vagi and the intestinal enteric nervous system.They are the same nerves that
promote motility and secretion in other parts of the upper gastrointestinal tract.

In summary, the gallbladder empties its store of concentrated bile into the duodenum mainly in response
to the cholecystokinin 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.