question from aamc 3R

[bugsbugsbugs]

can someone help me w/ questions 195 and 196 in the biological sciences section? i think i'm missing something

i thought plasma clearance was the basically how well you can remove stuff from plasma, so if ur rate of absorption goes up, then so does plasma clearance. seems like the solutions are saying that a decrease in plasma clearance is defined by an increase in absorption by the kidney (seems backwards)

i'm confused

i'm pretty sure i'm missing something. any help is appreciated

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[Prophecies]

Question 195:
When blood pressure is lower than normal, less fluid (filtrate) is entering the kidney to be filtered. As a result, the kidney has more time to absorb what it can from what little fluid there is. Hence, more of Substance A will be absorbed back into the body and its clearance by the kidney will decrease. Therefore, the answer is C.

Another way--none of the other answers make sense:
A --> wrong because the lower the blood pressure, the less Substance A can be filtered out by the kidney because less fluid is entering the kidney to begin with
B --> although it is true that ADH levels will be very low (because the blood volume is so low so the body will want to conserve fluids), no ADH will result in more concentrated urine (less water) but will not affect the reabsorption of Substance A, just the concentration of urine
D --> ADH levels increase in patients with hypotension; ADH levels decrease in patients with HYPERTENSION because the body wants to remove as much excess fluid from the blood as possible

Question 196:
Looking at the graph, you can see that at 8 mg/mL on the x-axis (plasma concentration) the glucose curve does not even exist. That means that the concentration in the urine has to be 0 mg/mL, meaning that glucose has not reached its Tm and will not be cleared from the kidney.

However, at 8 mg/mL Substance A has a concentration of about 2 mg/mL in the urine so it is obviously being cleared from the kidney. Since glucose isn't being cleared but Substance A IS, answer choice B is correct.

That is my understanding of those two questions. Good luck!

 

[bugsbugsbugs]

but the passage defines plasma clearance as "the capacity of the kidney to remove a substance from the plasma", not the rate at which substances leave the kidney

 

[SilvrGrey330]

D --> ADH levels go down with hypotension and increase with hypertension (high blood pressure) because the body wants to remove excess blood volume by urinary excretion


Wouldnt ADH (since its job is to re-absorb water or retain it) be lower in patients with Hypertension, because they already have a high BP as it is anyways, why would the body exacerbate the situation?
And therefore if ADH is suppresed, u will urinate more and ur BP will return to normal...or do i have it all backwards?

 

[Prophecies]

Yes, I am sorry. ADH levels increase in patients with hypotension, so the body can conserve precious blood volume, and decrease in patients with hypertension so the body can remove excess fluid from the blood and lower pressure. I have corrected my statements...sorry!

 

[Prophecies]

but the passage defines plasma clearance as "the capacity of the kidney to remove a substance from the plasma", not the rate at which substances leave the kidney

Yes, you are right. My explanations are based upon plasma clearance (the capacity of the kidney to remove substances from plasma), as well as reabsorption. I do not base my answers on the rate at which substances leave the kidney. Does my explanation above make sense?

 

[keedz]

I have a question: how good is the 3R as an indication of where you are?

 

[Prophecies]

I have a question: how good is the 3R as an indication of where you are?

I don't think the 3R is the best way to gauge what you will score on the actual MCAT, since it is an older version of the test. The best way to go would be to purchase 7R from www.e-mcat.com, since 7R was from an administration two years ago (2003).

That being said, 3R is still an ACTUAL MCAT from some point and time, so it is probably better than any Kaplan or Princeton test you will find. 3R will give you a rough estimate of the numbers you can expect on April 16.