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In Renal failure ... ur kidney dont work ... and unable to absorb Na+ .. thus more sodium excrete ... means fraction of Na+ ecxretion increase then normal ....
In Pre-renal failure .... kidney is Ok .. but getting less blood supply due to dehydration or heart failure or other cause .... as kidney gets less blood supply activates renin angiotensin aldosteron system .... thus increase aldosteron action causes more Na+ reabsorption then normal .... thus excretion of Na+ decreases then normal .....

In RF - ald cant work due to renal defect - Increase Na+ fraction excretion
In PRF - more ald due to hypoperfusion of kidney - More Na+ reabs = Decrease urinary Na+ fraction ........
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