BILIRUBIN METABOLISM | Anatomy2Medicine
BILIRUBIN METABOLISM

BILIRUBIN METABOLISM

  • Average life span red blood cells – 120 days (MCQ)
  • The hemoglobin is first split into globin and heme
  • heme ring is opened to give
    • free iron, which is transported in the blood by transferrin
    • a straight chain of four pyrrole nuclei, which is the substrate from which bilirubin will eventually be formed
  • The first substance formed is biliverdin, but this is rapidly reduced to free bilirubin,
  • The free bilirubin immediately combines strongly with plasma albumin and is transported in this combination throughout the blood and interstitial fluids
  • In liver cells
    • Bilirubin is conjugated about
      • 80 per cent with glucuronic acid to form bilirubin glucuronide,
      • about 10 per cent with sulfate to form bilirubin sulfate
      • about 10 per cent with a multitude of other substances
    • the bilirubin is excreted from the hepatocytes by an active transport process into the bile canaliculi and then into the intestines.
  • Formation and Fate of Urobilinogen.
    • Once in the intestine, about half of the “conjugated” bilirubin is converted by bacterial action into the substance urobilinogen, which is highly soluble. (MCQ)
    • Some of the urobilinogen is reabsorbed through the intestinal mucosa back into the blood.
    • Most of this is re-excreted by the liver back into the gut, but about 5 per cent is excreted by the kidneys into the urine.
    • After exposure to air in the urine, the urobilinogen becomes oxidized to urobilin; alternatively, in the feces, it becomes altered and oxidized to form stercobilin..

Diagnostic Differences Between Hemolytic and Obstructive Jaundice.

  • In hemolytic jaundice, almost all the bilirubin is in the “free” form
  • in obstructive jaundice, bilirubin is mainly in the “conjugated” form.
  • When there is total obstruction of bile flow
    • no bilirubin can reach the intestines to be converted into urobilinogen by bacteria. Therefore, no urobilinogen is reabsorbed into the blood,
    • none can be excreted by the kidneys into the urine.
    • Consequently, in total obstructive jaundice, tests for urobilinogen in the urine are completely negative(MCQ)
    • the stools become clay colored owing to a lack of stercobilin and other bile pigments. (MCQ)
  • Another major difference between free and conjugated bilirubin
    • kidneys can excrete small quantities of the highly soluble conjugated bilirubin but not the albumin-bound free bilirubin. (MCQ)
    • Therefore, in severe obstructive jaundice, significant quantities of conjugated bilirubin appear in the urine. (MCQ)
    • This can be demonstrated simply by shaking the urine and observing the foam, which turns an intense yellow. (MCQ)