DIGESTIVE SYSTEMLIVER

PANCREAS

Blood supply to the liver

I. The hepatic portal vein (hpv) and the hepatic artery send blood to the liver.

II. Their branches form the interlobular vessels which branch into distributing vessels that are located at the periphery of the lobule.

III. The interlobular vessels that form the smallest portal triads send blood into the sinusoids.

IV. In the sinusoids the blood flows centripetally toward the central vein.

V. The central vein courses through the central axis of the classic liver lobule and empties into a sublobular vein.

VI. 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 acinusThe 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 ACINUSThe 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 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.

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.

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

HepatocytesHepatocytes 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 lipids2. 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.

LysosomesLysosomes concentrated near the bile

canaliculus correspond to the peribiliary dense bodies. They have varied contents including:

-pigment granules (lipofuscin)-myelin figures-partially digested cytoplasmic

organellesmay 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.

PANCREAS

The exocrine component of pancreasThe 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

I. 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.

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 cellsThey 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 isletsThe PP cells secrete pancreatic polypeptide.

It stimulates gastric chief cellsIt inhibits bile secretion and intestinal motility.

It inhibits pancreatic enzymes and bicarbonate secretion.

The minor cells of isletsThe 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.