FUNCTIONAL ANATOMY OF LIVER AND BILIARY SYSTEM
Liver is a dual organ having both
secretory and excretory functions. It is the largest gland in the body,
weighing about 1.5 kg in man. It is located in the upper and right side of the
abdominal cavity, immediately beneath diaphragm.
LIVER
Hepatic Lobes
Liver is made up of many lobes called
hepatic lobes. Each lobe consists of many lobules called
hepatic lobules.
Hepatic Lobules
Hepatic lobule is the structural and functional
unit of liver. There are about 50,000 to 100,000 lobules in the liver. The
lobule is a honeycomb-like
structure and
it is made up of liver cells called hepatocytes.
Hepatocytes and
Hepatic Plates
Hepatocytes are arranged in columns,
which form the hepatic plates. Each plate is made up of two columns of cells.
In between the two columns of each plate lies a bile canaliculus .
In between the neighboring plates, a
blood space called sinusoid
is
present. Sinusoid is lined by the
endothelial cells. In between the
endothelial cells some special macrophages called Kupffer cells are present.
Portal Triads
Each lobule is surrounded by many
portal triads. Each portal triad consists of three vessels:
1. A branch of hepatic artery
2. A branch of portal vein
3. A tributary of bile duct.
Branches of hepatic artery and portal
vein open into the sinusoid. Sinusoid opens into the central vein.
Central vein empties into hepatic
vein. Bile is secreted by hepatic cells and emptied into bile canaliculus. From canaliculus, the
bile enters the tributary of bile duct. Tributaries of bile duct from canaliculi
of neighboring lobules unite to form small bile ducts. These small
bile ducts join together and finally
form left and right hepatic ducts,
which emerge out of liver.
BILIARY SYSTEM
Biliary system or extrahepatic biliary
apparatus is
formed by gallbladder and extrahepatic bile ducts (bile
ducts outside the liver). Right and
left hepatic
bile ducts which
come out of liver join to form common hepatic duct. It unites with the cystic duct from gallbladder to form common bile duct. All these ducts
have similar structures. Common bile duct
unites with pancreatic duct to form the common hepatopancreatic duct or ampulla of Vater, which opens into the
duodenum.
There
is a sphincter called sphincter
of Oddi at the lower part of
common bile duct, before it joins the pancreatic duct. It is formed
by smooth muscle fibers of common bile duct. It is normally kept closed; so the bile secreted from liver
enters gallbladder where it is stored. Upon appropriate stimulation, the sphincter
opens and
allows flow of bile from gallbladder into the intestine.
BLOOD SUPPLY TO LIVER
Liver receives maximum blood supply of
about 1,500 mL/minute. It receives blood from two sources, namely the hepatic
artery and portal vein.
HEPATIC ARTERY
Hepatic artery arises directly from
aorta and supplies oxygenated
blood to
liver.
PORTAL VEIN
Portal vein is formed by superior
mesenteric vein and splenic vein. It brings deoxygenated blood from stomach, intestine,
spleen and pancreas. Portal blood is rich in monosaccharides and amino acids.
It also contains bile salts, bilirubin, urobilinogen and GI hormones. However, the
oxygen content is less in portal blood. Flow of blood from intestine to liver
through portal vein is known as enterohepatic circulation.
The blood from hepatic artery mixes
with blood from
portal vein in hepatic sinusoids. Hepatic cells obtain
oxygen and nutrients from the
sinusoid.
HEPATIC VEIN
Substances synthesized by hepatic
cells, waste products and carbon dioxide are discharged into sinusoids. Sinusoids drain them
into central
vein of
the lobule. Central veins from many lobules unite to form bigger veins, which
ultimately form hepatic veins (right and left) which open into inferior vena cava.
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