Pre renal, renal and post renal failure..plz help - got2study2pass

[ArchivalUser]

hi, can u please help me on this...


tell me the diff between pre renal and renal and post renal.


i was thinking that if it is renal..the BUN/CR shud be more than 20:1, because the kidney is not functioning and cannot excrete the urea so its in serum and ratio goes up....

and in pre renal since less blood is going to the kidney so the bun/cr ratio shud also be more than 20:1 right. cud someone fix my mistake on this.

[ArchivalUser]

Prerenal, renal, and postrenal azotemia
Azotemia refers to an increase in serum BUN and creatinine.

----------Prerenal azotemia
Caused by a decrease in cardiac output
Hypoperfusion of the kidneys decreases GFR.
There is no intrinsic renal parenchymal disease.
Examples-blood loss, congestive heart failure
Serum BUN:Cr ratio greater than 15

-----------Renal azotemia (uremia)
Caused by parenchymal damage to the kidneys
Examples-acute tubular necrosis, chronic renal failure
Serum BUN:Cr ratio is 15 or below. b.c no compensatory increase in BUN reabsorption

-----------Postrenal azotemia
Caused by urinary tract obstruction below the kidneys
No intrinsic parenchymal disease
Examples-prostate hyperplasia, blockage of ureters by stones/cancer
Serum BUN:Cr ratio greater than 15
--above from Goljan Rapid Review Pathology

Pre-renal and post-renal azotemia are considered relatively benign and potentially reversible, If you can correct hypoperfusion of the kidney or relieve the obstruction, renal function will back to normal; otherwise both will lead to renal damage, so renal azotemia.

[ArchivalUser]

great, thanks so much juicy...just one more question

the increased BUN/cr ratio in pre renal...

is that because less blood is going to kidneys thus less being excreted..

[ArchivalUser]

less renal plasma flow --> less filtration --> flow in tubular system slow, which facilitates BUN reabsorption --> serum BUN does up further: BUN/Cr >15.

This compensatory increase in tubular BUN reabsorption is lost in renal azotemia, b.c. renal tubular dysfunction in RENAL azotemia, so BUN/Cr

[ArchivalUser]

But Juicy, why is there a BUN reabsorption...

isnt the bun/cr ratio increased because..less blood is going to the kidney thus less Creat is excreted..thus ratio goes up...

[ArchivalUser]

Pre renal ARF........hypovolemia thus......increased reabsp of water and sodium in PCT.........this is accompanied by inc urea reabsp from PCT..........thus BUN/Cr is more than 20......thus FeNa is also less than 1

whereas for ATN.......it is tubular defect thus sodium and urea reabsp is affected simultaneously....inc Na and urea lost in the urine.......thus BUN/Cr is less than 20 and FeNa more than 1

[ArchivalUser]

wow thanks waynerooney...one more question

isnt Cr reabosorbed. so u mean that urea is reabsorbed more than the creatinine thus ratio increases...

thanks

[ArchivalUser]

Hey got2study2pass,

I don't know why, but I cann't seem to post a complete message today-let's me try one more time. I see that waynerooney has provided a detailed explanation. Again, put it in a simple way, pre-renal azotemia--> renal plasma flow is low --> filtration is low --> which means BUN and Cr filtration is low --> serum BUN and Cr is up (azotemia)--> additionally, remember Cr usually is not reaborbed but BUN will be reabsorbed in renal tubule --> the slower the flow, the more will reabsorbed --> so more BUN will be reabsorbed in pre-renal azotemia --> this drives up the BUN/Cr ratio to >15. This increase in tubular BUN reabsorption will not happen in renal azotemia b.c. the loss of tubular function there, so BUN/Cr ratio remain low at

[ArchivalUser]

Creatinine is never reabsorbed..........just a little secretion thus best test of kidney function is calculation of creatinine clearance

Innulin clearance is best test to calculate GFR as no secretion and no reabsp but clinically not used........as innulin is an exogenous substance whereas creatinine is from diet and muscle

[ArchivalUser]

Hey got2study2pass: still an incomplete posting! but most infor has been posted...hope it helps. waynerooney mentioned that BUN reabsorption is coupled to Na+, which is very useful too as urine sodium level/excretion fraction is another important parameter for determining the source of azotemia.. hope it helps