Dark urine and intra vs extravascular hemolysis

[ChessMaster3000]

Maybe I missed something huge when I first learned this, but Rx seems to make a point that you only get dark urine from hemoglobin due to INTRAvascular hemolysis. Two questions about this:

1. The dark urine is due only to the hemoglobin? Does bilirubin have anything to do with it?
2. Why not extravascular hemolysis? Perhaps the macrophages consume rbcs in a more organized way in the spleen, but there is still an increased bilirubin load on the liver.

One other semi-related question that I have never been able to wrap my head around. Why do pigment gallstones due to chronic hemolysis contain UNconjugated bilirubin? I thought it can't get into the canaliculus unless its conjugated. Where is the uncojugated bill in pigment stones coming from?

Many thanks.

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[Apoplexy__]

Yes yes yes, I love this topic. I was so pumped when I fully understood the distinction.

A summary of the mechanisms:
Intravascular hemolysis --> Hemoglobin release in the blood (haptoglobin levels drop as it tries to capture free Hgb) --> Hemoglobinuria (dark urine)
Extravascular hemolysis --> Buildup of Hgb breakdown products --> conjugating systems of liver are overwhelmed --> excess unconjugated bilirubin leaks into blood --> Unconjugated bilirubinemia

Now, understand that hemoglobin --> bilirubin occurs only inside macrophages. Review http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Heme_Breakdown.png/250px-Heme_Breakdown.png So when RBCs lyse while in vessels, they're going to drop what they're currently carrying, which is Hgb. There's no way that the monocytes in the blood are going to grab all that Hgb, break it down, then somehow release it back into the blood instead of the liver.

Answer to #1: Yes, it's only the Hgb. There should be no bilirubin, especially in an acute hemolytic episode.
Answer to #2: See above. Extravascular hemolysis occurs solely because macrophages are killing more RBCs. So you can see why bilirubin would accumulate, not Hgb.

Answer to semi-related question: Yes, the bilirubin passing through the canaliculi is predominantly conjugated, and only trace amounts of unconjugated get in. But you can see how if you chronically have tons of unconjugated coming in through the canaliculi, that small amount becomes non-negligible, particularly since unconjugated bilirubin is so insoluble and is just dying to precipitate out. As for brown pigment stones in infection, that's probably more due to the bacteria deconjugating bilirubin back into unconjugated, just like they do in the intestines to produce urobilin.

The Goljan Hematology lectures do a great job of explaining this too. I highly recommend them.

 

[ulikedaggers]

Maybe I missed something huge when I first learned this, but Rx seems to make a point that you only get dark urine from hemoglobin due to INTRAvascular hemolysis. Two questions about this:

1. The dark urine is due only to the hemoglobin? Does bilirubin have anything to do with it?
2. Why not extravascular hemolysis? Perhaps the macrophages consume rbcs in a more organized way in the spleen, but there is still an increased bilirubin load on the liver.

One other semi-related question that I have never been able to wrap my head around. Why do pigment gallstones due to chronic hemolysis contain UNconjugated bilirubin? I thought it can't get into the canaliculus unless its conjugated. Where is the uncojugated bill in pigment stones coming from?

Many thanks.

You need a significant amount of hemolysis (chronic hemolysis) to overload the liver -> increased bilirubin in urine.

Intravascular hemolysis (in PNH, for example) results in hemoglobinuria following saturation of haptoglobin. This doesn't occur in extravascular hemolysis because the reticuloendothelial system is active. Anyway, once haptoglobin is saturated, free Hb gets filtered and ends up in the urine.

 

[ChessMaster3000]

Yes yes yes, I love this topic. I was so pumped when I fully understood the distinction.

A summary of the mechanisms:
Intravascular hemolysis --> Hemoglobin release in the blood (haptoglobin levels drop as it tries to capture free Hgb) --> Hemoglobinuria (dark urine)
Extravascular hemolysis --> Buildup of Hgb breakdown products --> conjugating systems of liver are overwhelmed --> excess unconjugated bilirubin leaks into blood --> Unconjugated bilirubinemia

Now, understand that hemoglobin --> bilirubin occurs only inside macrophages. Review http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Heme_Breakdown.png/250px-Heme_Breakdown.png So when RBCs lyse while in vessels, they're going to drop what they're currently carrying, which is Hgb. There's no way that the monocytes in the blood are going to grab all that Hgb, break it down, then somehow release it back into the blood instead of the liver.

Answer to #1: Yes, it's only the Hgb. There should be no bilirubin, especially in an acute hemolytic episode.
Answer to #2: See above. Extravascular hemolysis occurs solely because macrophages are killing more RBCs. So you can see why bilirubin would accumulate, not Hgb.

Answer to semi-related question: Yes, the bilirubin passing through the canaliculi is predominantly conjugated, and only trace amounts of unconjugated get in. But you can see how if you chronically have tons of unconjugated coming in through the canaliculi, that small amount becomes non-negligible, particularly since unconjugated bilirubin is so insoluble and is just dying to precipitate out. As for brown pigment stones in infection, that's probably more due to the bacteria deconjugating bilirubin back into unconjugated, just like they do in the intestines to produce urobilin.

The Goljan Hematology lectures do a great job of explaining this too. I highly recommend them.

Got it. Great explanation. I think the key is that conversion to bilirubin only happens inside macrophages--do you know how they get from the macrophages to the hepatocytes? There is no part of the process where the bilirubin LEAVES the macrophage and circulates bound to albumin on its way to the hepatocyte? (Because if there was, then some of it would show up in the urine as well, I presume)

 

[alicealicealice]

In extravascular hemolysis the macrophages are indeed secreting unconjugated bilirubin into the blood. But unconjugated bilirubin is not water soluble and is tightly bound to albumin, so it is not susceptible to glomerular filtration. Instead, these patients can get jaundice while the unconjugated bili takes its trip to the liver from the spleen (just as it does in normal RBC destruction every 120 days, now theres just alot more of it). Eventually the (healthy) liver catches up to this and the unconjugated bili gets processed in the normal way.

Its easy to get confused between hemoglobin and bilirubin in hemolysis, but neither extra- nor intravascular hemolysis produces bilirubinuria. To get bilirubin in the urine it needs to be conjugated, and at no point in hemolysis does conjugated bilirubin accumulate.

As an aside, to get conjugated bili in the blood something needs to be wrong with the hepatobiliary system. The most common causes of bilirubinuria are hepatocellular or cholestatic disease, and in the ddx are the rare inherited defects of conjugated excretion, such as Rotor and Dubin–Johnson's syndrome as well as drug side effects. Gilbert and Crigler–Najjar produce unconjugated hyperbilirubinemia hence no real bilirubinuria.

Agreed with the above poster who said Goljan's lecture on this was great. These might help:

Intravascular:

Extravascular:

 

[ChessMaster3000]

Yes yes yes, I love this topic. I was so pumped when I fully understood the distinction.

A summary of the mechanisms:
Intravascular hemolysis --> Hemoglobin release in the blood (haptoglobin levels drop as it tries to capture free Hgb) --> Hemoglobinuria (dark urine)
Extravascular hemolysis --> Buildup of Hgb breakdown products --> conjugating systems of liver are overwhelmed --> excess unconjugated bilirubin leaks into blood --> Unconjugated bilirubinemia

Now, understand that hemoglobin --> bilirubin occurs only inside macrophages. Review http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Heme_Breakdown.png/250px-Heme_Breakdown.png So when RBCs lyse while in vessels, they're going to drop what they're currently carrying, which is Hgb. There's no way that the monocytes in the blood are going to grab all that Hgb, break it down, then somehow release it back into the blood instead of the liver.

Answer to #1: Yes, it's only the Hgb. There should be no bilirubin, especially in an acute hemolytic episode.
Answer to #2: See above. Extravascular hemolysis occurs solely because macrophages are killing more RBCs. So you can see why bilirubin would accumulate, not Hgb.

Answer to semi-related question: Yes, the bilirubin passing through the canaliculi is predominantly conjugated, and only trace amounts of unconjugated get in. But you can see how if you chronically have tons of unconjugated coming in through the canaliculi, that small amount becomes non-negligible, particularly since unconjugated bilirubin is so insoluble and is just dying to precipitate out. As for brown pigment stones in infection, that's probably more due to the bacteria deconjugating bilirubin back into unconjugated, just like they do in the intestines to produce urobilin.

The Goljan Hematology lectures do a great job of explaining this too. I highly recommend them.

One other thing I always had trouble with was that in the gut lumen, when bacteria deconjugate to urobilin/urobilinogen, and this gets taken up via enterohepatic, how does that urobilin/urobilinogen get into the urine? It was deconjugated, so it should no longer be soluble in water and should be bound to albumin. OR is it that even thought its uncojugated, it was still metabolized in such a way that it is still water soluble?

 

[Apoplexy__]

Got it. Great explanation. I think the key is that conversion to bilirubin only happens inside macrophages--do you know how they get from the macrophages to the hepatocytes? There is no part of the process where the bilirubin LEAVES the macrophage and circulates bound to albumin on its way to the hepatocyte? (Because if there was, then some of it would show up in the urine as well, I presume)

alicealicealice's post answers your question very well.

OR is it that even thought its uncojugated, it was still metabolized in such a way that it is still water soluble?

Yes, exactly. When conj. bilirubin gets converted into sterco/urobilin, it becomes soluble again. A small amount (the minority) of it gets absorbed via portal circulation, and can enter the kidneys after reaching general circulation.

The next step in the bilirubin discussion is to be able to use a UA to explain the etiology of bilirubinemia. Another mind-blowing thing for me is that you can determine the etiology of a bilirubinemia solely with the bilirubin and urobilin in a UA. Huge note to re-emphasize: unconjugated bilirubin never shows up in the urine. So in the breakdown below, "bilirubin" = conjugated bilirubin only.

Intra-hepatic UA findings (hepatitis):
Bilirubin +
Urobilin +
Post-hepatic UA findings (cholelithiasis):
Bilirubin +
Urobilin (-)
Pre-hepatic UA findings (hemolysis):
Bilirubin (-)
Urobilin +

See if you can logic this out, and if it all makes sense to you. Remember that urobilin is formed only from conjugated bilirubin, so if there's something preventing the gut from receiving conjugated bilirubin, urobilin won't accumulate in the urine.

 

[ChessMaster3000]

alicealicealice's post answers your question very well.



Yes, exactly. When conj. bilirubin gets converted into sterco/urobilin, it becomes soluble again. A small amount (the minority) of it gets absorbed via portal circulation, and can enter the kidneys after reaching general circulation.

The next step in the bilirubin discussion is to be able to use a UA to explain the etiology of bilirubinemia. Another mind-blowing thing for me is that you can determine the etiology of a bilirubinemia solely with the bilirubin and urobilin in a UA. Huge note to re-emphasize: unconjugated bilirubin never shows up in the urine. So in the breakdown below, "bilirubin" = conjugated bilirubin only.

Intra-hepatic UA findings (hepatitis):
Bilirubin +
Urobilin +
Post-hepatic UA findings (cholelithiasis):
Bilirubin +
Urobilin (-)
Pre-hepatic UA findings (hemolysis):
Bilirubin (-)
Urobilin +

See if you can logic this out, and if it all makes sense to you. Remember that urobilin is formed only from conjugated bilirubin, so if there's something preventing the gut from receiving conjugated bilirubin, urobilin won't accumulate in the urine.

Awesome. So two things to clarify--for our purposes, urobilin=urobilinogen? And also, for the intrahepatic situation, I get the urobilin, but I'm not sure I under the bilirubin. As in, the liver conjugates it but there is so much hepatic damage it can't properly direct all of it to the canalicular system so some of it goes back out into the blood?

Lastly, could you imagine a situation with simultaneous bilirubinuria and hemoglobinuria? I'm trying to think of one right now and can't come up with anything.

 

[Apoplexy__]

Awesome. So two things to clarify--for our purposes, urobilin=urobilinogen?

Yeah. The verbage of stercobilin/ogen and urobilin/ogen is pretty useless for all tests (school or boards). All four terms are effectively the same. I'm sure you can Google some place that goes into detail about it but it's just confusing and useless.

And also, for the intrahepatic situation, I get the urobilin, but I'm not sure I under the bilirubin. As in, the liver conjugates it but there is so much hepatic damage it can't properly direct all of it to the canalicular system so some of it goes back out into the blood?

Yeah pretty much. The primary pathology is the liver being unable to receive incoming bilirubin from the bloodstream, while the conjugating abilities are fine. So it can only take on a smaller amount of the bilirubin coming in from the systemic circulation, which it conjugates fine and sends out into the bile. Although there's less bilirubin arriving in the intestines since less was passed through the liver, this too eventually accumulates because what gets absorbed in portal circulation gets roadblocked again when it comes back to the liver.

Lastly, could you imagine a situation with simultaneous bilirubinuria and hemoglobinuria? I'm trying to think of one right now and can't come up with anything.

Hm...I can't think of one. Mostly since, as I'm feeling like you understand, bilirubinuria (aka conj. bilirubinemia) and hemoglobinuria are really pathophysiologically distinct. But you can have simultaneous unconj. bilirubinemia and hemoglobinuria in things that simultaneously set off both mechanisms of intravascular and extravascular hemolysis (e.g. malaria is an example). That's not very high-yield though.

 

[ChessMaster3000]

Yeah. The verbage of stercobilin/ogen and urobilin/ogen is pretty useless for all tests (school or boards). All four terms are effectively the same. I'm sure you can Google some place that goes into detail about it but it's just confusing and useless.



Yeah pretty much. The primary pathology is the liver being unable to receive incoming bilirubin from the bloodstream, while the conjugating abilities are fine. So it can only take on a smaller amount of the bilirubin coming in from the systemic circulation, which it conjugates fine and sends out into the bile. Although there's less bilirubin arriving in the intestines since less was passed through the liver, this too eventually accumulates because what gets absorbed in portal circulation gets roadblocked again when it comes back to the liver.



Hm...I can't think of one. Mostly since, as I'm feeling like you understand, bilirubinuria (aka conj. bilirubinemia) and hemoglobinuria are really pathophysiologically distinct. But you can have simultaneous unconj. bilirubinemia and hemoglobinuria in things that simultaneously set off both mechanisms of intravascular and extravascular hemolysis (e.g. malaria is an example). That's not very high-yield though.

Alright. I'll lay it to rest. Hepatobiliary is usually one of my strongest subjects by far, so I'm glad I got that filled in because its just a few minor points that makes everything else fall into place. Thanks a lot.

 

[Apoplexy__]

Alright. I'll lay it to rest. Hepatobiliary is usually one of my strongest subjects by far, so I'm glad I got that filled in because its just a few minor points that makes everything else fall into place. Thanks a lot.

Yeah, I hear you. Happy to help!

 

[alicealicealice]

Maybe I missed something huge when I first learned this, but Rx seems to make a point that you only get dark urine from hemoglobin due to INTRAvascular hemolysis. Two questions about this:
Why do pigment gallstones due to chronic hemolysis contain UNconjugated bilirubin? I thought it can't get into the canaliculus unless its conjugated. Where is the uncojugated bill in pigment stones coming from?

Many thanks.

Listening to pathoma, he mentions that bacteria can deconjugate the bilirubin in the gall bladder, particularly if stasis is present, leading to pigment stones.

 

[ChessMaster3000]

Listening to pathoma, he mentions that bacteria can deconjugate the bilirubin in the gall bladder, particularly if stasis is present, leading to pigment stones.

Yes, that's right--thanks to beta-glucoronidase enzyme in bacteria.

 

[ChessMaster3000]

Hey so I came across something in pathoma that said ineffective erythropoeisis can also cause an unconjugated bilirubinemia. @kirbymiester I was wondering if you knew anything more about this, as in, when the erythrocyte precursors are destroyed in the bone marrow, is that analogous to extravascular hemolysis because there must be some monocytes in the bone marrow (it's basically like the spleen?). and do you know if they are autolysed or something or do the macrophages detect their inadequacy and consume them? I'd guess the latter

 

[Apoplexy__]

Hey so I came across something in pathoma that said ineffective erythropoeisis can also cause an unconjugated bilirubinemia. @kirbymiester I was wondering if you knew anything more about this, as in, when the erythrocyte precursors are destroyed in the bone marrow, is that analogous to extravascular hemolysis because there must be some monocytes in the bone marrow (it's basically like the spleen?). and do you know if they are autolysed or something or do the macrophages detect their inadequacy and consume them? I'd guess the latter

I'm 99% sure that's exactly it. Taken from https://www.inkling.com/read/histology-michael-ross-and-wojciech-pawlina-6th/chapter-10/bone-marrow :

The cords also contain macrophages, mast cells, and some adipose cells. Although the cords of hemopoietic tissue appear to be unorganized, specific types of blood cells develop in nests or clusters. Each nest in which erythrocytes develop contains a macrophage. These nests are located near the sinusoid wall.

The only question is if these macrophages are just the ones giving iron to the developing RBCs (that is where the RBCs first get their iron; remember where iron is hidden away in anemia of chronic disease), or if they're also doing quality-control phagocytosis. I think it's safe to assume they reach their pseudopods through the sinusoids and "feel around" the RBCs to screen them just like they do in the spleen. Assuming it is analogous then, this hemolysis would be from phagocytosis.

Even if the mechanism of who exactly is destroying these RBCs and how the hemoglobin is disposed of is a little cloudy, you know just from the fact that ineffective hematopoiesis can cause unconj bilirubinemia that it must be extravascular. Incidentally, it's that kind of skillful inference that you showed that will take you far in boards -- grasping commonly known processes well enough that you can infer into the dark.

 

[ChessMaster3000]

I'm 99% sure that's exactly it. Taken from https://www.inkling.com/read/histology-michael-ross-and-wojciech-pawlina-6th/chapter-10/bone-marrow :

The cords also contain macrophages, mast cells, and some adipose cells. Although the cords of hemopoietic tissue appear to be unorganized, specific types of blood cells develop in nests or clusters. Each nest in which erythrocytes develop contains a macrophage. These nests are located near the sinusoid wall.

The only question is if these macrophages are just the ones giving iron to the developing RBCs (that is where the RBCs first get their iron; remember where iron is hidden away in anemia of chronic disease), or if they're also doing quality-control phagocytosis. I think it's safe to assume they reach their pseudopods through the sinusoids and "feel around" the RBCs to screen them just like they do in the spleen. Assuming it is analogous then, this hemolysis would be from phagocytosis.

Even if the mechanism of who exactly is destroying these RBCs and how the hemoglobin is disposed of is a little cloudy, you know just from the fact that ineffective hematopoiesis can cause unconj bilirubinemia that it must be extravascular. Incidentally, it's that kind of skillful inference that you showed that will take you far in boards -- grasping commonly known processes well enough that you can infer into the dark.

Great--it was only from your help thru our first discussion that I would have been able to make that inference. Only since then have I done several questions on that topic and realized how much i did not understand before. But I agree, extrapolation is key before the actual test, as it is incredibly uncomfortable to extrapolate without practice during the actual test.

 

[Apoplexy__]

Awesome. Good to hear.