SIADH and [Na+] along the nephron

[PagingDr.F]

During SIADH/antidiuresis, what site of the nephron has the highest concentration of Na+?

Since there is reabsorption but no secretion of Na+ along the nephron, where would it be highest? Is it in the descending loop of henle (concentrating segment) or would it be in the collecting duct, even though almost 99% of the filtered Na+ has already been reabsorbed by this point?

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[FindersFee5]

This is an interesting question - I would assume in the collecting duct, because that's the only place that's being affected by the ADH, which seems to be what this question is testing.

The other part that plays into this - SIADH causes hypotonicity, so the Na+ concentration in previous parts of the nephron are going to be lower than normal, and will continue to be lower than normal until it reaches the distal tubule/collecting ducts where water is reabsorbed by the increased aquaporin channels.