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Old 04-08-2012
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Urine Sample High urine osmolarity in prerenal failure!

what is the cause of high urine osmolarity in pre-renal failure? main determinant of urine osmolarity is sodium, it is less in prerenal failure...
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Default my first thought

I'm not sure if this is the only answer but my first thought was:

hypovolemia-> stimulation of renin-angiotensin-aldosterone-> increase of Na concentration
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Old 04-08-2012
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Default sorry for confussing

sorry for confusing you... I just realized I misread the question. please discard my previous post.
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Old 04-09-2012
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Urine with high osmolality reflects two things:
1. hypovolemic-stimulus to the secretion of ADH
2. maintenance of normal tubular function

Common tendency is towards low sodium concentration (<20 meq/L)

Although in combination with low water excretion, urine would have relatively higher concentration of Na+ and high osmolality
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Old 04-14-2012
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In prerenal syndrome, the osmolality is high, but not from sodium - the UNa is low in prerenal. The kidney is reabsorbing sodium preferentially because of low perfusion/RAA. Then it is reabsorbing water at the collecting ducts because of high ADH. The result is a urine with high osmolality but a low sodium.
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Old 04-14-2012
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Quote:
Originally Posted by Mondoshawan View Post
In prerenal syndrome, the osmolality is high, but not from sodium - the UNa is low in prerenal. The kidney is reabsorbing sodium preferentially because of low perfusion/RAA. Then it is reabsorbing water at the collecting ducts because of high ADH. The result is a urine with high osmolality but a low sodium.
I'm sorry, but I'm a little confused. I understand that the volume of urine produced is lower, but why is the osmolarity high if the amount of solute is also lower? Should it be equimolar?
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because urine carries lots of stuff other than Na. a highly concentrated urine can have low Na and high osmolarity because of Urea and other crap

is how I see it
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Old 04-14-2012
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in prerenal failure and acute glomerularnephritis the tubular function of the kidney is preserved
whereas in ATN there is loss of this tubular function
they can be differentiated by the urine sediment analysis
in ATN u will see RBC casts and hematuria
Check in Goljan path renal section for more detailed explantion
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Old 06-09-2012
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Quote:
Originally Posted by tyagee View Post
what is the cause of high urine osmolarity in pre-renal failure? main determinant of urine osmolarity is sodium, it is less in prerenal failure...
In pre renal there is a decrease in cardiac output
This produces decrease in renal plasma flow
this increases filtration fraction
this increases the oncotic pressure in the peritubular capillaries
this increase reabsorption of water
hence osmotic pressure of urine increases
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