Download DIGESTIVE SYSTEM

LIVER
PANCREAS
Blood supply to the liver
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VI.
The hepatic portal vein (hpv) and the hepatic artery send
blood to the liver.
Their branches form the interlobular vessels which
branch into distributing vessels that are located at the
periphery of the lobule.
The interlobular vessels that form the smallest portal
triads send blood into the sinusoids.
In the sinusoids the blood flows centripetally toward the
central vein.
The central vein courses through the central axis of the
classic liver lobule and empties into a sublobular vein.
Several sublobular veins converge to form larger hepatic
veins that empty into the inferior vena cava
BLOOD SUPPLY TO THE LIVER
DIAGRAM OF A “CLASSIC” LOBULE
Liver acinus
The liver acinus described as diamond shaped, is
the smallest functional unit in the hepatic
parenchyma.
The short axis of the acinus is defined by the
terminal branches of the portal triad that lie along
the border between two “classic” lobules.
The long axis is a line drawn between the two
central veins closest to the short axis.
The liver acinus provides the best correlation
among blood perfusion, metabolic activity, and
liver pathology.
DIAGRAM OF A LIVER ACINUS
The cells in zone 1 are the
first to receive both nutrients
and toxins in the blood and
are the first to show
morphologic
changes
following bile stasis. These
cells are also the last to die
and the first to regenerate.
The hepatocytes in each liver acinus are
described as being arranged in three concentric
elliptical zones surrounding the short axis.
Thus,
Zone 1 is closest to the axis. Zone 2 is farthest
from the axis. Zone 2 lies between zones 1 and 3.
The cells in zone 3, on the
other hand, are the first to
show
ischemic
necrosis.
They are last to respond to
toxic substances and to bile
stasis
The cells in zone 2 have all
characteristic between zones
1 and 3.
Sinusoids
Hepatic sinusoids are lined with a thin
discontinuous endothelium.
Hepatic sinusoids have Kupffer cell or the
stellate sinusoidal macrophage.
The Kupffer cells may be involved in the final
breakdown of some damaged or senile red
blood cells that reach the liver from the spleen.
Schematic diagram of a plate of hepatocytes
interposed between hepatic sinusoids
Hepatocytes
Hepatocytes make up the anastomosing cell
plates of the liver lobule.
Hepatocytes are large polygonal cells that
constitute about 80% of the cell population
of the liver.
Nuclei of hepatocytes are large and
spherical and occupy the center of the cell.
Liver cells are capable of considerable
regeneration when liver substance is lost to
disease or surgery.
Peroxisomes functions:
1. Specific oxidative functions in:
-gluconeogenesis
-metabolism of purines
-metabolism of alcohol
-metabolism of lipids
2. About one-half of the ethanol that one might
drink is converted to acetaldehyde by enzymes
contained in liver peroxisomes
3. In humans catalase and D-amino acid oxidase,
as well as alcohol dehydrogenase are found in
peroxisomes.
Lysosomes
Lysosomes concentrated near the bile
canaliculus correspond to the peribiliary
dense bodies. They have varied contents
including:
-pigment granules (lipofuscin)
-myelin figures
-partially digested cytoplasmic organelles
may be a normal storage site for iron.
Biliary tree
I. The bile canaliculus is small canal formed by
grooves in neighboring cells.
II. The bile canaliculi form a ring about the
hepatocyte and drains into small bile ducts, the
canals of Hering, these in turn, drain into the
bile duct of the portal canals.
III. Bile flows in the canaliculi centrifugal. The flow
of bile is in a direction opposite to the flow of
blood, i.e. from the region of the central vein
toward the portal canal.
Diagram of the blood and bile flow
in the liver
Bile
Structure:
About 90% of the bile salts a component of
bile is reabsorbed and resecreted by the
hepatocytes.
Bile also consists of cholesterol, lecithin,
bile pigments, water and electrolytes.
The exocrine component of pancreas
The exocrine portion of the pancreas is a compound
acinar gland with serous type of secretion
The initial portion of the intercalated duct begins
within the acinus.
The intercalated duct cells located inside the acinus
are called centroacinar cells.
Cells which composed pancreatic acini are called
acinar cells.
They are characterized by acidophilic zymogen
granules.
Diagram of a pancreatic acinus
and its duct system
Duct system
The intercalated ducts are short and
drain to intralobular collecting ducts.
II. The network of intralobular ducts drains
to the larger interlobular ducts.
III. The interlobular ducts, in turn, drain
directly into the main pancreatic duct.
IV. A second large duct, the ductus
choledochus (accessory pancreatic duct)
arises in the head of the pancreas.
I.
Endocrine pancreas
Structure:
The islets of Langerhans, the endocrine
component of the pancreas, are scattered
throughout the organ.
The islets constitute about 1-2% of the
volume of the pancreas.
The B cells
They constitute about 70% of the total islet
cells.
They are generally located in its central
portion.
Functions:
They secrete insulin which stimulate:
uptake of glucose from the circulation
utilization and storage of glucose by all cells
The D cells
They constitute about 5-10% of the total
pancreatic endocrine tissue.
They are also located peripherally in the islets.
Functions:
They secrete somatostatin which inhibit:
insulin and glucagon secretion
Diagram of an islet of Langerhans
The minor cells of islets
The PP cells secrete pancreatic
polypeptide.
It stimulates gastric chief cells
It inhibits bile secretion and intestinal
motility.
It inhibits pancreatic enzymes and
bicarbonate secretion.
The minor cells of islets
The EC cells secrete secretin, motilin and
substance P.
Secretin acts locally to stimulate
bicarbonate secretion in pancreatic fluid
and pancreatic enzyme secretion.
Motilin increases gastric and intestinal
motility.
Substance P function is unclear.