Obstructive jaundice and dark urine

[Mcat35]

can someone explain to me why Biliary tract obstruction (obstructive jaundice) can produce Dark urine (due to bilirubinuria).

My thought process Conjugated bilirubin is unable to get into the gallbladder, so it is backed up in the blood results in increased renal excretion of conjugated bilirubin? Is this correct?

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[thawunandonly]

I think that's how I understood it. Conjugated bilirubin is water soluble.

 

[loveoforganic2]

I think that's how I understood it. Conjugated bilirubin is water soluble.

Yep, sounds right to me

 

[iBS1972]

This is correct. A quick review of the enterohepatic bilirubin circulation:
heme->biliverdin->bilirubin->to liver->conjugation to bile (aka conjugated bilirubin)->secreted into duodenum->broken down in large intestine by bacteria->urobilinogen->stercobilinogen OR resorbed into bloodstream and oxidized to urobilin which gives urine its color.

Now urobilin gives urine the yellow color. Sterobilin gives poop the brown color.
In obstructive jaundice, you don't have secretion of conjugated bilirubin (bile) into the GI tract, so the bacteria in the large intestine does not have any bilirubin to breakdown into sterobilin which is why you get pale stool!
So why do you still get dark urine? Note: you also don't get any urobilin either. BUT the buildup of conjugated bilirubin leaks into the urine. Bilirubin is also pigmented! bili-rubin (rubin for red, which is why you get dark urine).