Could somebody help me understand BUN:Cr ratios?

[studylol]

Hello friends,

I don't really understand Dr Sattar's explanation. Specifically, i'm trying to understand why BUN:Cr ratios change under different circumstances:

a. Prerenal azotemia.

b. Postrenal azotemia.

What I know: BUN can be filtered and reabsorbed. Cr is filtered and never reabsorbed. So BUN is primarily what changes during different conditions. Dr Sattar says that when Na and water is reabsorbed, such as with prerenal azotemia, then BUN follows it. Does this happen to elevate serum osmolarity beyond what Na is capable of doing alone?

thanks very much 🙂

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[addidas]

As you said BUN is filtered and reabsorbed. Cr is filtered but not reabsorbed.
a. Prerenal: Low flow into the kidney -> GFR is low -> Urine travels through the system slower = more water and Na is reabsored (RAAS) and so is BUN
Cr meanwhile flows through and out.
In this case there is more BUN because less is getting to the kidney and more is being reabsorbed. Cr is more because less is getting to the kidney.
So BUN increases more than Creatnine = ratio increases.

b. Intrarenal:
Flow to kidney is normal ->BUT damage to the tubular cells means GFR is low.
Damage to cells means that the BUN which wouldve been reabsorbed isn't and flows out with Cr.
As GFR is decreased both BUN and Cr increase but their ratio stays the same as BUN isn't absorbed.

c. Postrenal: essentially behaves like Prerenal at first except this time GFR is low due to less urine flowing out because of back pressure- urine stays in the system longer hence more BUN is reabsorbed back.
Later you get damage to the cells and it starts behaving like intrarenal failure.

 

[studylol]

Do you know why BUN is reabsorbed more in prerenal azotemia?

EDIT: I think the answer is because since GFR decreases, the filtrate moves more slowly throughout nephron, giving it more time to be reabsorbed.