Page 554 - Small Animal Internal Medicine, 6th Edition

526 PART IV Hepatobiliary and Exocrine Pancreatic Disorders

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A B

FIG 33.6
Jaundiced mucous membranes in a dog (A, gum; B, sclera). Note that this dog had
jaundice because of immune-mediated hemolytic anemia and not liver disease—hence, the
mucous membranes are pale and yellow, which makes them more easily photographed.
(Courtesy Sara Gould.)

canaliculi. Conjugated bilirubin is then incorporated into Portal triad
micelles and stored with other bile constituents in the gall-
bladder until it is discharged into the duodenum. However,
in dogs, only 29% to 53% of bile produced is stored in the
gallbladder; the rest is secreted directly into the duodenum.
After arrival in the intestine, conjugated bilirubin undergoes
Zone
bacterial deconjugation and then reduction to urobilinogen, Central Zone 1 2
with most urobilinogen being resorbed into the enterohe- vein
patic circulation. A small fraction of urobilinogen is then Zone
3
excreted in the urine, and a small portion remains in the
intestinal tract to be converted to stercobilin, which imparts
normal fecal color.
Inherited abnormalities of bilirubin metabolism have not
been identified in cats and dogs, so in the absence of massive
increases in bile pigment production by hemolysis, jaundice
is attributable to impaired excretion of bilirubin, and usually FIG 33.7
Rappaport scheme of the hepatic functional lobule (acinus),
other constituents of bile, by diffuse intrahepatic hepatocel- organized according to biochemical considerations (1958).
lular or biliary disease or by interrupted delivery of bile to This is centered on a line connecting two portal triads and
the duodenum. The inability to take up, process intracel- describes functional zones radiating from the triad to the
lularly, or excrete bilirubin into the bile canaliculi (the rate- central vein. For example, zone 1 cells are responsible for
limiting step) is the mechanism of cholestasis believed to be protein synthesis, urea and cholesterol production,
operational in many primary hepatocellular diseases. Jaun- gluconeogenesis, bile formation, and β oxidation of fatty
acids; zone 2 cells also produce albumin and are actively
dice is more likely to be a clinical feature if the liver disorder involved in glycolysis and pigment formation; and zone 3
primarily involves the periportal (zone 1) hepatocytes (Fig. cells are the major site of liponeogenesis, ketogenesis, and
33.7) than if the lesion involves centrilobular (zone 3) hepa- drug metabolism. Zone 3 hepatocytes, being farther from
tocytes. Inflammation and swelling of larger intrahepatic the hepatic artery and hepatic portal veins, also have the
biliary structures could similarly delay bile excretion. lowest oxygen supply and are therefore most susceptible to
Extrahepatic biliary duct obstruction (EBDO) occurs as hypoxic damage. Arrows show direction of blood flow. The
a result of intraluminal or extraluminal obstruction of the portal triad comprises one or more branches of bile duct
(green), hepatic artery (red), and hepatic portal vein (violet).
bile duct at any point along its length. This may be obstruc-
tion of the cystic duct by a gallbladder mucocele in dogs or
obstruction of the common bile duct in dogs and cats by tocellular regurgitation of bile constituents into the circu-
choleliths, neoplasia of occasionally foreign bodies. Jaundice lation, and jaundice. If only one of the hepatic bile ducts
can develop in both cats and dogs as a result of acute flare-up exiting the liver is blocked, or if only the cystic duct exiting
of a chronic pancreatitis causing transient extrahepatic the gallbladder is obstructed for some reason, there may
biliary obstruction. Indeed, an acute flare-up of chronic pan- be biochemical clues for localized cholestasis, such as high
creatitis is the commonest cause of EBDO in dogs. serum alkaline phosphatase activity; however, the liver’s
Obstruction of the bile duct near the duodenum results overall ability to excrete is preserved, and jaundice may
in increased intraluminal biliary tract pressure, interhepa- not ensue.