ADH Hormone Question on AAMC 3

[Dr Gerrard]

Here is the question. Why does a more than normal concentration of ADH not increase water absorption by the tubules? It claims that C and D are the same, but ADH, according to EK, acts in the distal tubule and the collecting duct, not in the loop of henle. Water absorption in the loop of henle is due to a sodium concentration gradient.

Also, at the end of the explanation, it makes the claim that water absorption would decrease, but that is not even the function of ADH?

I mean i understand how glomerular filtration rate might increase, thuse there would be a greater urine output, but what about everything else?

I don't think you actually need the passage to answer this question.

In Experiment 2, the increased blood pressure resulting from the higher-than-normal concentration of ADH most likely affected the urinary output of Substance A by increasing the:

A. glomerular filtration rate.
(The best answer is that increased blood pressure will affect the glomerular filtration rate, answer choice A. Tm is a characteristic that depends on the characteristics of the cells lining the renal tubules and independent of blood pressure, so answer choice B is not correct. Water resorption and concentrating ability are the same, so answer choices C and D are essentially the same. Increasing blood pressure should increase flow of fluid through the kidney system and decrease, rather than increase, water reabsorption, so these answer choices are incorrect.)

B Tm of solutes.
C water reabsorption from the tubules.
D concentrating ability of the loop of Henle.

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[Dr Gerrard]

Also, whats up with this passage. It says a vvolunteer was given one dose of normal level ADH, plus another dose of higher than normal (thus resulting in the above question), increasing the blood pressure.

I thought hormones result from condition changes. Condition changes do not result from hormones. Is it only this latter, opposite way in this question because the patient was administered the hormone?

So what happens when we have low blood pressure?

ADH levels must increase in order to raise our blood pressure, correct?
At this point, is our urine output lower or greater?

Maybe it is lower because the ADH can only do one function, either raise the BP or increase water absorption?

 

[Dr Gerrard]

Oh I think I just got it.

The question asks specifically about Substance A. Water reabsorption does not directly affect amount of Substance A, but glomerular filtration rate does, which is why the answer is A. Correct?

Still though, why does AAMC say increase in blood pressure due to ADH would decrease water reabsorption?

 

[Chowdder]

I think the question is not saying that ADH does not increase reabsorption, but rather that the pressure increase caused by ADH (due to increase reabsorption) will inhibit further reabsorption. That's negative feedback. Imagine if you have increase pressure that further increase reabsorption.....you'll be nothing but water