Page 438 - Concise Pathology for Exam Preparation ( PDFDrive )

15 Diseases of the Hepatobiliary System and Pancreas 423

Q. Write briefly on bilirubin metabolism.

Ans. Metabolism of Bilirubin (Flowchart 15.1):
• 80–85% of bilirubin is derived from the catabolism of the haemoglobin of senescent red
blood cells.
• 15–20% is derived from the bone marrow, destruction of maturing cells, liver and the
turnover of haem and haem-containing precursors (cytochromes, myoglobin, etc.).

RBCs Splitting of globin Amino acid pool

Haem
Haem oxygenase
Protoporphyrin + iron

Biliverdin
Biliverdin reductase
Unconjugated bilirubin + albumin

Hepatic phase Unconjugated bilirubin
UDP glucuronyl transferase
Conjugated bilirubin (mono or diglucuronide)
Canalicular transport system (rate-limiting step)
Conjugated bilirubin in common bile duct (excretion into bile)
Stored and concentrated in the gallbladder
Conjugated bilirubin in terminal ileum
Bacterial reduction by colonic bacteria
Stercobilinogen
Enterohepatic circulation
(a small amount of stercobilinogen is absorbed in the bowel;
passes through the liver and is excreted in the urine as urobilinogen)

Stercobilin Urobilinogen

Stool Urine
FLOWCHART 15.1. Bilirubin metabolism.

Q. Define and classify jaundice.

Ans. Bilirubin and cholesterol have low water solubility and cannot be excreted into urine.
Bile is the primary pathway for elimination of both. Hepatocellular damage leads to a
disruption in bile metabolism and manifests clinically as jaundice (yellowish pigmentation
of skin and mucous membranes) and icterus (yellow discoloration of sclera). The latter
occurs because bilirubin has a special affinity for elastin which is present abundantly in
the sclera. Yellow discoloration is also prominent in the palpebral conjunctiva, sublingual
mucosa and lower abdominal skin.

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