Confused over a renal question

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kwanny

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this question is from UW. it's hard to explain it, so for those of you who have done UW already, can you help me clarify this concept? I am more confused abou tthe explanation than the answer. BTW, only 20% of people got this question right on UW.

a patient receiving hydrochlorothiazide therapy is placed on low sodium diet. his urine sodium excretion follow the pattern depicted below:

[graph shows as dietary Na (y axis) going down as urinary Na excretion goes down in a hemi-"U"-shape].

The question asks to explain the pattern shwon in the graph. the answer was that it was due to increased reabsorption of Na in the proximal tubules.

The reasoning was that as dietary Na goes down, ECF & blood volume go down, thus activating RAAS system, which leads to increase in GFR, and thus leading to increased Na reabsorption.

My confusion is this: even if more Na is reabsorbed, it's a constant fraction that's being reabsorbed (67%) in the prox tubule, so the amount of Na excreted should also be elevated, and the fraction of Na in urine should stay fixed not lower.

I thought it was increase in aldosterone that made this difference, not increased reabsorption of in the proximal tubules.

Can someone kindly explain how their explanation makes any sense? Thanks guys.

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