Renal ischaemia question

[Phloston]

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For some reason, I remember from Goljan audios (unless I remember things incorrectly) that the medulla of the kidney receives only about 5-10% of the organ's blood supply, and therefore it is most susceptible to ischaemia. Goljan had also compared this to centrilobular necrosis in the liver, where zone-3 receives lesser oxygenation and therefore is most susceptible to ischaemia. It had also been my understanding that in chronic analgesic nephropathy, the medulla sloughs off first because of its sensitivity to medication-induced ischaemic change.

In GT, they say that the cortex, not the medulla, is most susceptible to ischaemia because most of the metabolically active, ATP-utilizing PCTs are there.

Could someone please comment on this?

Cheers,

 

[beeitchness]

S3 segment of outer medulla most susceptible to ischemia is what I remember

Proximal tubules are especially affected there.

 

[teenmachinery1]

Mr. pathoma says: "Proximal tubule and medullary segment of the thick ascending limb are
particularly susceptible to ischemic damage." So... both.

 

[HelpPleaseMD]

For some reason, I remember from Goljan audios (unless I remember things incorrectly) that the medulla of the kidney receives only about 5-10% of the organ's blood supply, and therefore it is most susceptible to ischaemia. Goljan had also compared this to centrilobular necrosis in the liver, where zone-3 receives lesser oxygenation and therefore is most susceptible to ischaemia. It had also been my understanding that in chronic analgesic nephropathy, the medulla sloughs off first because of its sensitivity to medication-induced ischaemic change.

In GT, they say that the cortex, not the medulla, is most susceptible to ischaemia because most of the metabolically active, ATP-utilizing PCTs are there.

Could someone please comment on this?

Cheers,

I just went through a UWORLD question on this an hr ago. The proximal and thick ascending limb are most susceptible to ischemic damage because they are the ones most involved in active transport and because of their location in the medulla which has a lower blood supply

 

[Go Ducks]

I'm just throwing this out there because I don't have a source to back it up...

Medulla has less perfusion because blood comes from the cortex and goes inward into the medulla, so it is near the ends of the blood supply. That puts the interstitium of the medulla at greater risk than the interstitium of the cortex.

However in terms of paranchyma, the PCT and thick ascending limb are most at risk due to high ATP use.

Therefore I believe that "cortex (PCT)" and "medulla" are both correct answers... but to slightly different questions.

 

[teenmachinery1]

PCT's in the cortex, not the medulla.

 

[Go Ducks]

I just went through a UWORLD question on this an hr ago. The proximal and thick ascending limb are most susceptible to ischemic damage because they are the ones most involved in active transport and because of their location in the medulla which has a lower blood supply

By proximal, do you mean proximal convoluted tubule? Because the PCT is in the cortex.

EDIT: never mind, teenmachinery1 beat me to pointing this out. Slow typist here.

 

[beeitchness]

By proximal, do you mean proximal convoluted tubule? Because the PCT is in the cortex.

EDIT: never mind, teenmachinery1 beat me to pointing this out. Slow typist here.

PCT in cortex, but straight proximal tubules in medulla

 

[HelpPleaseMD]

By proximal, do you mean proximal convoluted tubule? Because the PCT is in the cortex.

EDIT: never mind, teenmachinery1 beat me to pointing this out. Slow typist here.

Maybe I misinterpreted UWORLD? "Proximal tubule and thick ascending ascending limb of henle's loop are located in the outer medulla of the kidney, an area under even normal circumstances have low blood supply" ID. 885

 

[Go Ducks]

PCT in cortex, but straight proximal tubules in medulla

Yes, didn't think of that. When I saw "proximal", my mind went straight to PCT. Do you know if the proximal straight tubule is at significantly increased risk for ischemia? I've just heard insterstitium of medulla, the PCT, and the ascending limb listed as the worst areas.

 

[beeitchness]

Yes, didn't think of that. When I saw "proximal", my mind went straight to PCT. Do you know if the proximal straight tubule is at significantly increased risk for ischemia? I've just heard insterstitium of medulla, the PCT, and the ascending limb listed as the worst areas.

I said earlier up above that S3 segment of outer medulla is most susceptible of all areas in the kidney to ischemia

The proximal straight tubules there are the very first to go, usually

 

[Phloston]

This is copy and pasted directly from GT:

"A 25-year-old individual injures his right renal artery during a motor vehicle accident. If this injury results in a 50% decrease of his renal blood supply in the right kidney, which part of the kidney will be most at risk for ischemia?
A. Neither cortex nor medulla will be at risk due to collateral flows
B. Medulla
C. Initially the cortex, but subsequently the medulla will be most at risk
D. Cortex
E. Both cortex and medulla will be equally at risk

Answer Explanation

The correct answer is D.

The renal blood supply is normally 20% of cardiac output. 90-95% of renal blood flow goes to the cortex; the renal medulla receives only 5-10% of renal blood flow. The renal cortex houses the most metabolically active cells of the kidney - namely the proximal tubules. These cells require ATP for proper function. Despite the disparity between blood supply, because of the energy requirements by the proximal tubule cells, the renal cortex is more vulnerable to ischemic damage."

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As a group here, I'm not so sure we've come to any definitive answer yet. However, these GT questions are from recent graduates/students, aren't they? I've already encountered a few others that I've thought were a bit iffy.

Another one was which drug to use in a patient with HIT. I put "direct thrombin inhibitor," but the answer was "all of the above," and one of the answer choices was warfarin. I was just thinking, "warfarin isn't the first Tx, even if it is eventually used."

 

[Chirurg]

don't know if this will add to the confusion:

"Necrosis is inconspicuous and restricted to the highly susceptible outer medullary regions."

" Even under normal conditions, the medullary region subsists on a hypoxic precipice as a result of low blood flow and countercurrent exchange of oxygen, although paradoxically housing nephron segments with very high energy requirements (e.g., the S3 segment of the proximal tubule and the medullary thick ascending limb of Henle’s loop)."


http://jasn.asnjournals.org/content/17/6/1503.short#ref-13

more insight welcome.

 

[Phloston]

I think I may have figured this out..

If you notice, this GT question mentions an acute reduction in blood flow to the kidney. In this case, the cortex may be more greatly affected similar to how it also is with diffuse cortical necrosis secondary to septic shock or an obstetric catastrophe. My guess is that in the acutely hypoxic state, it's not that the medulla isn't susceptible as much as it is that the highly ATP-driven processes of the cortex are damaged first.

In the chronically hypoxic state, the absolute loss of oxygen is not great enough such that the ATP-driven processes of the cortex are hindered, but it is substantial enough such that the medulla, which already receives meager blood supply, merely atrophies over time.

Therefore:

Acute, severe hypoxia --> cortical necrosis, with loss of the PCTs + thick ascending limb
Chronic, sub-severe hypoxia --> medullary/papillary necrosis

(I actually just annotated those last two lines of my conclusion into FA)