MDpride

Full Member
10+ Year Member
5+ Year Member
May 28, 2007
680
3
Status (Visible)
  1. Medical Student
Hii

RR says that extrarenal loss of urea takes place causing BUN/CREATININE ratio to be < 15 before entering kidney in RENAL AZOTEMIA.

Why is extrarenal loss of urea not happening in PRERENAL AZOTEMIA?
 

Miss Alyssa

Member
10+ Year Member
Mar 10, 2005
620
8
Status (Visible)
RR says that prerenal azotemia is caused by a decreased CO --> decreased GRF --> back up of urea and creatinine in the blood. So both urea and creatinine are increased. Of the urea and creatinine that get filtered, creatinine cannot be reabsorbed but urea can. The decreased flow allows more urea to be reabsorbed further increasing the urea in the blood (and increasing the urea:creatinine)
 

MDpride

Full Member
10+ Year Member
5+ Year Member
May 28, 2007
680
3
Status (Visible)
  1. Medical Student
RR says that prerenal azotemia is caused by a decreased CO --> decreased GRF --> back up of urea and creatinine in the blood. So both urea and creatinine are increased. Of the urea and creatinine that get filtered, creatinine cannot be reabsorbed but urea can. The decreased flow allows more urea to be reabsorbed further increasing the urea in the blood (and increasing the urea:creatinine)

because there is back up of urea and creatinine,there is more time for extrarenal urea loss?

so there should be some factor that determines when extrarenal loss of urea will take place?
 

Sora

Sic transit gloria
10+ Year Member
Dec 10, 2007
117
0
Status (Visible)
  1. Medical Student
There are factors that regulate extrarenal WATER loss to conserve fluid in contracted states, which will therefore affect extrarenal UREA loss (remember urea is amphiphilic), including ADH. When you get a back-up of urea and creatinine in PRErenal azotemia, you are volume contracted and will therefore have decreased extrarenal loss of water (and urea). In renal azotemia, you are volume expanded, and will get increased extrarenal water loss, and therefore increased extrarenal urea loss.

However, I wouldn't worry about any of this for the boards. It does seem like anything that would be emphasized. I think the key is that renal azotemia means BUN:creatinine =< 15.
 
About the Ads

Rabbit Hole

We're all mad here.
Dec 29, 2009
526
7
Wonderland
Status (Visible)
  1. Medical Student
Hii

RR says that extrarenal loss of urea takes place causing BUN/CREATININE ratio to be < 15 before entering kidney in RENAL AZOTEMIA.

Why is extrarenal loss of urea not happening in PRERENAL AZOTEMIA?
BUN:Creatinine is a ratio.
Normal BUN = 10
Normal Cr = 1
Normal BUN:Cr = 10:1

Prerenal azotemia BUN:Cr approximately 15:1
- Only BUN level is elevated
- You got low blood flow and have extra time to pick up some urea.

Renal failure BUN:Cr approximately 10:1
- Both BUN and Cr levels are elevated
- You got a problem with your glomeruli so both urea and creatinine get elevated.


.. or something like that.
(Goljan talks about this in his audio)
 

rem6775

Full Member
7+ Year Member
Mar 7, 2010
205
3
Status (Visible)
  1. Resident [Any Field]
BUN:Creatinine is a ratio.
Normal BUN = 10
Normal Cr = 1
Normal BUN:Cr = 10:1

Prerenal azotemia BUN:Cr approximately 15:1
- Only BUN level is elevated
- You got low blood flow and have extra time to pick up some urea.

Renal failure BUN:Cr approximately 10:1
- Both BUN and Cr levels are elevated
- You got a problem with your glomeruli so both urea and creatinine get elevated.


.. or something like that.
(Goljan talks about this in his audio)

That part is not true. In prerenal azotemia both Cr and BUN are increased, but BUN is increased more so than Cr because it is reabsorbed by the kidney (and Cr cannot be reabsorbed). They are both elevated, however.
 

MDpride

Full Member
10+ Year Member
5+ Year Member
May 28, 2007
680
3
Status (Visible)
  1. Medical Student
There are factors that regulate extrarenal WATER loss to conserve fluid in contracted states, which will therefore affect extrarenal UREA loss (remember urea is amphiphilic), including ADH. When you get a back-up of urea and creatinine in PRErenal azotemia, you are volume contracted and will therefore have decreased extrarenal loss of water (and urea). In renal azotemia, you are volume expanded, and will get increased extrarenal water loss, and therefore increased extrarenal urea loss.

However, I wouldn't worry about any of this for the boards. It does seem like anything that would be emphasized. I think the key is that renal azotemia means BUN:creatinine =< 15.


THANK YOU
Volume Contraction didn't come to mind. thats a logical explanation.
 

Super Mario

Full Member
10+ Year Member
Dec 13, 2008
20
0
Status (Visible)
  1. Resident [Any Field]
1) Pre-renal failure:
kidney senses decreased blood flow --> normal response is increased proximal water and sodium reabsorption --> urea passively follows H2O and Na --> therefore increased BUN relative to creatinine (ratio > 20) and fractional excretion of Na < 1% highly suggestive of prerenal failure

Caveat:Malnutrition (low protein intake) or chronic liver disease (low protein production) --> low urea --> BUN/creatinine may not be elevated --> BUN/creatinine <20 does not exclude prerenal failure

2) Intrinsic renal disease (ATN, AIN, glomerular disease): kidney not functioning properly --> unable to reabsorb H2O and Na properly --> since H2O and Na not being reabsorbed, urea does not follow passively --> BUN/creatinine < 20, FeNa > 1%

Hope that helps
 
This thread is more than 11 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.