free water clearance question

[MudPhud20XX]

One of Kaplan physio questions:

The inability to the kidneys to concentrate the urine will cause which of the following patterns of urine flow, urine osmolarity, extracellular fluid volume, and plasma osmolarity compared with a normal subject?

The answer was increased urine flow and decreased urine osmolarity.

Can anyone explain why you would necessarily get increased urine flow? I get the decreased urine osmolarity. But wouldn't also get decreased urine flow since this would be an opposite of diabetic diuresis where high urine osmlarity increases the urine flow, right?

Many thanks in advance.

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[Dharma]

sounds like diabetes insipidus. but by flow, they just mean flow rate, i.e. lots of pee!

next question you have to ask yourself: what will distinguish central vs nephrogenic?

 

[sazerac]

Just to clarify, the point Kaplan is trying to get across is that the kidney increases concentration of the urine by removing water, not by dumping in a lot of molecules. So if the kidney can't concentrate the urine, you are peeing out normal urine PLUS a lot of extra water.

 

[CherryRedDracul]

Just to clarify, the point Kaplan is trying to get across is that the kidney increases concentration of the urine by removing water, not by dumping in a lot of molecules. So if the kidney can't concentrate the urine, you are peeing out normal urine PLUS a lot of extra water.

This. Remember that body, under normal circumstances, wants to hold onto water so you don't have to interrupt your daily activities drinking water a million times a day. The purpose of ADH/vasopressin is to "Always Dig Holes" (yeah, ExamKrackers mnemonic!) in your distal convoluted tubules and collecting ducts so that precious water can be reabsorbed back into circulation. That, in turn, concentrates the filtrate so you get concentrated urine.

If you can't concentrate the urine by drawing water out of the filtrate, you're gonna lose all that precious water in your urine, hence why urine flow increases.