ANATOMY AND PHYSIOLOGY
THE BILIARY SYSTEM as part of the DIGESTIVE SYSTEM
The biliary system consists of the
organs
and
gallbladder,
ducts
(bile
and
ducts,
associated
structures) that are involved in the
production and transportation of bile.
The transportation of bile follows this
sequence:
When the liver cells secrete
bile, it is collected by a system
of ducts that flow from the liver
through
the
right
and
left
hepatic ducts.
These ducts ultimately drain
into the common hepatic duct.
The common hepatic duct then joins with the cystic duct from the gallbladder to
form the common bile duct, which runs from the liver to the duodenum (the first
section of the small intestine).
However, not all bile runs directly into the duodenum. About 50 percent of the bile
produced by the liver is first stored in the gallbladder, a pear-shaped organ
located directly below the liver.
Then, when food is eaten, the gallbladder contracts and releases stored bile into
the duodenum to help break down the fats.
The biliary system's main function includes the following:
To drain waste products from the liver into the duodenum
To help in digestion with the controlled release of bile
Bile is the greenish-yellow fluid (consisting of waste products, cholesterol, and
bile salts) that is secreted by the liver cells to perform two primary functions, including
the following:To carry away waste; and Tobreak down fats during digestion
Bile salt is the actual component which helps break down and absorb fats. Bile,
which is excreted from the body in the form of feces, is what gives feces its dark brown
color. Bile and bile salts are manufactured by the liver and fed into the second part of
the duodenum via the common duct.
Bile salts are important in fat digestion. The pancreas is a retro-peritoneal gland
that is both an endocrine - producing the hormones insulin and glucagon, and an
exocrine gland - producing digestive enzymes. The enzymes are secreted in a
deactivated form - to prevent auto-digestion - and are activated in the lumen of the
duodenum.
Gallbladder.The gallbladder is a pear-shaped muscular storage sac which holds
bile, situated below the right lobe of the liver. Bile secreted between meals flows from
the liver through the cystic duct into the gallbladder.
During a meal, the hormone cholecystokinin-pancreozymin causes the
gallbladder to contract and squeeze bile into the duodenum to help digest fats and
certain vitamins. After the food has been digested, the gallbladder relaxes and is ready
to store bile again.
Bile is continuously produced by the liver, and its viscosity changes according to
its chemical and excretory content. Bile flows easily when it is thin, and the gallbladder
serves as a good storage area so that it does not constantly drip into the duodenum
when no food is present.
Bile does not flow easily when it is thick, and the gallbladder helps to squirt
enough of it into the duodenum when food is present. In other words, the gallbladder
has a regulatory function to ensure that the right amount of bile is released into the
duodenum at the right time.
The biliary tree conducts bile and pancreatic digestive enzymes to the
duodenum. The gross anatomy of the biliary tree begins with the right and left hepatic
ducts that drain bile from the two halves of the liver. These become the common hepatic
duct that is joined by the cystic
duct from the gallbladder.
The
union of the common hepatic and
cystic ducts form the common
bile duct. The common bile duct
is about 7.5 cm long. It passes
posterior and often through the
pancreas
to
join
the
main
pancreatic duct (duct of Wirsung).
The union of the main pancreatic
duct and common bile duct form
short
ampula
called
hepatopancreaticampula
the
(a.k.a.
ampula of Vater).
The ampula inserts on the
major duodenal papilla, which is
guarded by the hepatopancreatic sphincter (a.k.a. sphincter of Oddi). A minor accessory
duct called Santorinis duct, when present may drain a portion of the pancreatic head
into the minor duodenal papilla. The accessory duct is not present in most individuals.
The physiology of the biliary tract causes bile to be concentrated in the
gallbladder in the absence of fat in the diet. Likewise, bile is released when fat and
some proteins are present in the diet. The mechanism for bile concentration, storage
and release is controlled primarily by the hormone cholecystokinin (CCK); other
hormones gastrin and secretin along with vagal stimulation play minor roles.
When the sphincter of Oddi is closed, hydrostatic pressure forces bile through
the cystic duct into the gallbladder (retrograde filling). When chyme-containing fat
reaches the duodenum, cells in the duodenum secrete CCK into the blood.
Cholecystokinin is a hormone that when it reaches the gallbladder it causes it to
contract.
The action of CCK on the duodenal sphincter is to relax allowing muscular
contractions of the gallbladder moves bile without resistance. The time from ingestion of
lipids to stimulation of the gallbladder to contract is roughly 30 minutes. Complete
emptying of the gallbladder takes about 1 hour. The hormones CCK, gastrin and
secretin are cholesecretagogues. A secretagogue is a substance that stimulates
secretion. Cholesecretagogues stimulate secretion of bile by the gallbladder.
