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Discussion Starter · #1 ·
Interesting question:
A man was recently diagnosed with SIADH. Which part of the renal tubule would have most highly concentrated urine?

Options:
a. Proximal convoluted tubule
b. Distal convoluted tubule
c. collecting duct- cortical portion
d. collecting duct- medullary portion
e. Loop of henle

Answer with at least one point of justification
 

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Option D

I'd go with option D.

In case of SIADH there will be high level of plasma ADH. This hormone increases absorption of water ( not solutes ) mainly from collecting duct.

As the tubular filtrate goes through proximal tubule, the absorption here will be isoosmotic i.e., concentration of the filtrate won't change.

Reaching ascending lope of Henle, there will be a high amount of absorption of solutes i.e., sodium, potassium, chloride, calcium...etc but not water because this segment is inpermeable to free water. This activity will make the filtrate dilute.

In distal tubule, there will be absorption sodium which will be accompanied by free water....

Finally in collecting tubule, the ADH works to absorb free water only... This final action will make the filtrate further concentrated.... To clarify the fact of selecting medullary collecting ducts, I think ( although am not certainly sure ) there is a system called counter current exchange system and this will make the filtrate even further concentrated....
 

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Discussion Starter · #3 ·
Very good answer....Regarding Medullary CD what I read was ADH has maximal action here and very little on DCT and the cortical portion of CD>
That's why in the presence of ADH, Collecting duct (medullary portion) has maximal urine concentration.

In the absence of ADH however, The loop of Henle, the portion between the descending and ascending loops is maximally hypertonic.
 

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yes that is correct ..
in case of SIADH the most hypertonic part is Medullary part of Collecting DUCT..
while in the absence of SIADH or normal ADH the loop of Henle has the greatest concentration...
 
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