A question about bilirubin

[MedicineMan99]

Ok I have been going nuts trying to figure this concept out, I would greatly appreciate any input. I can't seem to find a source that makes it clear.

I understand this part:
Once conjugated bilirubin has been excreted in the bile, the gut bacteria create urobilinogen (colorless). This gets excreted in feces as stercobilin which makes feces brown and also in urine as urobilin which makes urine yellow.

Here's my question:
If someone presents with dark urine and clay-colored stool, how is that possible? If there was an obstruction of the biliary system, wouldn't that decrease the amount of urobilinogen which would then not only make feces clay-colored, but urine more clear? The only explanation I can come up with is that perhaps the default color of urine without bilirubin is darker than if it had bilirubin- but I don't think that concept is correct.

Thanks for any input!

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[Depakote]

Ok I have been going nuts trying to figure this concept out, I would greatly appreciate any input. I can't seem to find a source that makes it clear.

I understand this part:
Once conjugated bilirubin has been excreted in the bile, the gut bacteria create urobilinogen (colorless). This gets excreted in feces as stercobilin which makes feces brown and also in urine as urobilin which makes urine yellow.

Here's my question:
If someone presents with dark urine and clay-colored stool, how is that possible? If there was an obstruction of the biliary system, wouldn't that decrease the amount of urobilinogen which would then not only make feces clay-colored, but urine more clear? The only explanation I can come up with is that perhaps the default color of urine without bilirubin is darker than if it had bilirubin- but I don't think that concept is correct.

Thanks for any input!

p366 of RR Path if you want to follow along

Normal Phys:
Conjugated bilirubin ->bile->

Urobilinogen goes to Urobilin (aka stercobilin, they're the same thing) -> your poop is brown

Enterohepatic circulation-> reuptake of Urobilin-> kidney-> your pee is yellow


P369 (Rapid Review Path)
If you have an obstruction. No Urobilinogen in the bile (pale stool) and it's not there for enterohepatic reuptake so it never makes it back to the kidneys. But you do get a high conjugated bilirubinemia.

EDIT: just reread your question... DARK urine... hmm. don't know about that. have to look into that myself.

 

[Depakote]

yeah, the stuff I'm looking at, urine/stool get light/dark in tandem when it's caused by bilirubin.

What prompted you to ask this?

 

[bola]

I believe obstruction of the bile ducts can simply just lead to back up of conjugated bilirubin in the intrahepatic ducts, hepatocytes and eventual leakage into the blood stream due to damaged hepatocytes -hence the conjugated bilirubinemia. Since the bile is already conjugated, it is now soluble so it can be excreted by the kidneys hence causing dark urine

The bile never reaches the stool due to the biliary obstruction so the stool is clay colored.

 

[Cliff Huxtable]

Does this help?

"Jaundice due to extrahepatic bile duct obstruction.

Any obstruction of the bile duct between the liver and the duodenum, including stones in the common duct, cancer of the head of the pancreas and any pathology involving the Ampulla of Vater (but NOT stones in the gallbladder.)

In these conditions, liver function is normal, at least at the commencement, and bilirubin is conjugated. However, bilirubin cannot reach the intestine and so the water-soluble, conjugated bilirubin goes back into the blood plasma and reaches all tissues. The plasma bilirubin concentration can be greatly elevated and jaundice marked. Conjugated bilirubin is excreted into the urine which is dark brown, maybe almost black. Stools are pale. Once obstruction is complete then stools are off-white or grey and no urobilinogen can be formed so urobilinogen is absent from the urine. Longstanding extrahepatic bile duct obstruction eventually causes impairment of hepatocyte function also (see below)."

http://florey.biosci.uq.edu.au/GMC/GMCyr1L1.html


There is also a figure on that website:

I think the answer is that the stercobilin is not reaching the feces so it is colorless (which you know already).

The urobilin still reaches the urine because the excess bilirubin gets into the blood which then gets filtered by the kidneys, so urobilin is still found in the urine?

I don't know if this is right or not?

 

[MedicineMan99]

yeah, the stuff I'm looking at, urine/stool get light/dark in tandem when it's caused by bilirubin.

What prompted you to ask this?

Thanks for your reply. The question is from this practice exam:

http://download.usmle.org/2009step1.pdf

Question #4 on page 25.

I think I came up with another club (page 318 in FA 2009), which says that Direct bilirubin is water soluble and can be excreted into urine then a chart shows urine bilirubin as elevated in an obstructive jaundice and a decrease in urine urobilinogen. I think my problem was that I didn't know conjugated bilirubin can be directly excreted in urine without being transformed into urobilinogen. If urine bilirubin is still yellow, which I think it is, then I think it answers the question in how the urine is dark (excess yellow bilirubin). What do all of you think?

 

[Mortal_Lessons]

Thanks for your reply. The question is from this practice exam:

http://download.usmle.org/2009step1.pdf

Question #4 on page 25.

I think I came up with another club (page 318 in FA 2009), which says that Direct bilirubin is water soluble and can be excreted into urine then a chart shows urine bilirubin as elevated in an obstructive jaundice and a decrease in urine urobilinogen. I think my problem was that I didn't know conjugated bilirubin can be directly excreted in urine without being transformed into urobilinogen. If urine bilirubin is still yellow, which I think it is, then I think it answers the question in how the urine is dark (excess yellow bilirubin). What do all of you think?

This is very simple-please just follow along. If you have a biliary obstruction, bile (along with the bilirubin present in it) will not make its way to the GI tract and color the stool. The stool will be more clay-colored. The biliary obstruction will also result in a conjugated bilirubinemia-conjugated bilirubin, being water soluble, will filter in the kidney and make the urine dark (on the other hand, unconjugated bilirubin which is bound to albumin will not be filtered at the kidney-but this is not the case in a biliary obstructive scenario). The whole deal about urobilinogen is somewhat irrelevant to this discussion as the dark urine is a result of the conjugated bilirubin being filtered at the kidney...

 

[pseudoknot]

It's worth noting that according to Goljan, stercobilinogen and urobilinogen are exactly the same molecule, but just with different names depending on where they are.

 

[rabiya]

This is addressed in RR 369 with a nice picture on pg 368 . You get dark urine because "In obstructive liver disease, and INCREASE in serum and URINE CB is due to obstruction of intrahepatic and extrahepatic bile flow...this causes increased pressure in the inthrahepatic bile ductules leading to rupture and egress of CB into sunduoidal blood."

Thus because of the egress of CB into the serum, CB eventually makes it way to the kidney and you get dark urine. You get light colored stools because of the obstruction (CB cannot go into the intestine to be converted to UBG.

Goljan has a very nice picture of that on pg 368!

 

[Waysensei]

This has been confusing me as well. Here are my thoughts:

Dark urine and pale stools could be due to Hepatic Jaundice, i.e. hepatitis. There is diminished conjugation of bilirubin in the liver, so there is less conjugated bilirubin in the gut to be oxidized to urobilin, hence the pale stools. The urobilin in the gut is then redirected to the kidneys to be filtered into the urine since there is impaired liver reuptake via enterohepatic recirculation due to hepatocellular necrosis, so more urobilin reaches the urine and causes the dark urine. I understand this perfectly well.

What confuses me is how obstructive jaundice causes the dark urine. I have read the explanations that conjugated bilirubin produces color to the urine, but Goljan's RR Pathology states on page 358 that urobilin, not CB, produces the color of the urine. Urobilin is decreased in both the stool and the urine from the blockade, so shouldn't there be both pale stools and pale urine?

Ok I have been going nuts trying to figure this concept out, I would greatly appreciate any input. I can't seem to find a source that makes it clear.

I understand this part:
Once conjugated bilirubin has been excreted in the bile, the gut bacteria create urobilinogen (colorless). This gets excreted in feces as stercobilin which makes feces brown and also in urine as urobilin which makes urine yellow.

Here's my question:
If someone presents with dark urine and clay-colored stool, how is that possible? If there was an obstruction of the biliary system, wouldn't that decrease the amount of urobilinogen which would then not only make feces clay-colored, but urine more clear? The only explanation I can come up with is that perhaps the default color of urine without bilirubin is darker than if it had bilirubin- but I don't think that concept is correct.

Thanks for any input!

 

[Fuzuli]

Here's my question:
If someone presents with dark urine and clay-colored stool, how is that possible? If there was an obstruction of the biliary system, wouldn't that decrease the amount of urobilinogen which would then not only make feces clay-colored, but urine more clear? The only explanation I can come up with is that perhaps the default color of urine without bilirubin is darker than if it had bilirubin- but I don't think that concept is correct.

Thanks for any input!

Dark urine + light stools (bilirubinuria and acholic stools) is the classic presentation of obstructive jaundice, which is usually caused by choledocholithiasis. Let's go over it step-by-step:
1) Due to various reasos, there's gallstone formation in (not so suprisingly) the gallbladder (cholelithiasis). In about %15 of patients with cholelithiasis, gallstones pass to the common bile duct. [Note: Stones can also form de novo, but it's another topic of discussion]
2) Because of the obstruction, pressure in the biliary system rises, which causes progressive dilatation in intrahepatic bile ducts.
3) Due to this obstruction and dilatation, hepatic bile blow is suppressed.
4) So what happens to the conjugated bilirubin formed within the hepatocytes? Instead of being excreted in the bile, it passes to the bloodstream instead. Conjugated bilirubin then reaches to kidneys, where it's converted to urobilin and excreted in urine. Under normal circumstances, some of the bilirubin which has reached to the kidneys would have been excreted as stercobilin in the feces; but because of the obstruction, the excess bilirubin was transferred to kidneys instead. Thus, the amount of urobilin increases = bilirubinuria = dark urine
5) Because of the obstruction, bilirubin can't reach to the intestines = no bilirubin = no stercobilin = acholic stools

Hope this helps

 

[marktwain]

we fail to answer questions not because we dont know but because we think that because the answer is so obvious it can not be...the answer provided by mortal lessons was apt and nothing else need be added

 

[sedra55]

Hi
The explanation of dark urine is that; when biliary drainage is blocked, some of the conjugated bilirubin leaks out of the hepatocytes to blood and appears in the urine, turning it dark amber. Another explanation; could be the presence of hemoglobin in urine (hemoglobinuria) which occurs in hemolytic anemia. As you know in chronic hemolytic anemia there is increased risk of biliary stones and subsequently biliary obstruction with pale stool.