Renal System Question

[futuredoc331]

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I've taken two practice tests and they are each giving contradicting information regarding the affects of blood pressure on the renal system. The first test explained

"The best answer is answer choice C: that low blood pressure decreases the
glomerular filtration rate, allowing more time for reabsorption and decreasing the amount of substance A in
the urine."

The test I just finished says this is false.

"Since glomerular filtration rate (GFR) is directly proportional to blood pressure, increases in blood pressure would lead to increases in GFR. Therefore, if Steve’s blood pressure increased more than Joe’s, one would expect Steve’s GFR to increase more than Joe’s (choice D is true, and A is false). Reabsorption rate is not affected by blood pressure (choices B
and C are false)."

Which is it? Does Blood pressure affect re-absorption or only the filtration rate?

 

[piii]

Neither are wrong.

You need to look at the big picture of renal physiology.
There are 4 modes of transport:

Filtration - solutes/fluid passing from afferent arterioles into Bowman's space in the glomerulus, and into the tubule system.
Secretion - solutes/fluid passing from the blood in the efferent arterioles directly cross the tubule membrane into the renal lumen.
Reabsorption - solutes/fluid passing from renal tubule fluid into the capillaries.
Excretion - Elimination of solutes/fluid in the urine.

Now tying it together you have this formula:
Excretion = Filtration + Secretion - Reabsorption. This also applies to the rates.

If you isolate GFR, then filtration rate is directly proportional to blood pressure: increase in BP leads to an increase in filtration, and vice versa. This is due to hydrostatic pressure differences in the afferent arteriole versus Bowman's space.

The statement above is true in both answers.

Now, notice how the first answer says "decreasing the amount of substance A in the urine." If you decrease BP, and thus decrease filtration rate, you can infer that the rate of fluid passing through the tubule is slower, and thus you increase the time, or opportunity for the Ascending Limbs, DT/CD to access the fluid for reabsorption, ,assuming the substance is efficiently reabsorbed. Thus you decrease in urinary concentration.

Also, notice how the first answer mentions reabsorption, but doesn't say reabsorption rate. So while more is reabsorbed, the rate of reabsorption is not affected.

In the second answer, they make the distinction of reabsorption rate. This is not a function of blood flow, but a function of osmolarity gradient and transport systems within the tubule epithelial cells.

Hope that helps

 

[Axes]

Yeah, when they say reabsorption RATE that essentially means the rate of diffusion/transport, which we know must remain constant. But if low BP, then reabsorption can occur over a longer time period, which leads to more total reabsorption even at that constant rate. The main thing is just recognizing rate vs total. If you're a math guy you can think of it like derivative vs integral

 

[futuredoc331]

Thank you. That was very helpful.