Loop of Henle Question

[mjjdm1985]

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Long loops of Henle on juxtamedullary nephrons allow for greater concentration of urine. For an individual with highly concentrated urine, filtrate entering the loop of Henle is likely to be:

a. more concentrated than filtrate exiting the loop of Henle
b. less concentrated than filtrate exiting the loop of Henle
c. more voluminous than filtrate exiting the loop of Henle
d. less voluminous than filtrate exiting the loop of Henle

I answered C, but only because I read it too fast and thought it said "highly concentrated filtrate" instead of "highly concentrated urine." I still don't understand why A is actually the right answer, though.

My thoughts are once filtrate enters the loop of Henle it begins losing water during the descending loop because the membrane is permeable to water. This will concentrate the filtrate more. During the ascent the membrane is impermeable to water but permeable to Na+, which begins leaving. Is A the answer because sodium leaves the ascending loop at a faster rate than water leaves the descending loop?

Or is there something I'm not getting? Help! 🙂

 

[ACSheldor]

Well I'm guessing that since it comes in highly concentrated then not much water gets absorbed on the descending limb because of a higher than normal osmotic pressure in the descending limb. Then on the ascending limb, since it is so concentrated, as many Na ions that can be taken out are taken out. So in the end, you have next to no fluid taken out and a lot more Na ions than normal taken out making it less concentrated. Also remember that after the proximal tubule, the nephron is made for obtaining homeostasis, so more concentrated filtrate will be diluted. That's how I worked through it.

This sounds like a really tough question though. Where is it from?

 

[Instatewaiter]

Na is taken out, water is not so that means it is diluted at the end. Added to that the osmolarity is 300 going in and 100 in the early distal tubule and you have your answer.

 

[mjjdm1985]

You are both exactly right I just needed to come back to the question after taking a break. The "concentration of the urine" part threw me...I was thinking urine meant at the end of the process and I didn't see why it told us that. But filtrate is also known as primary urine isn't it? It seems much easier now.

And the problem is from ExamKrackers Bio Book 7th edition. It's part of the in-lesson questions for lesson 6.

Thanks!!

 

[ACSheldor]

Ahh, it sounded like a bad question, now that I know it came from EK I'm not surprised.

 

[mjjdm1985]

Haha yeah it seems like some of their questions are not that great, but it does seem like, for Bio at least, they do a fairly good job explaining everything.

Do you not feel like ExamKrackers is good preparation for the MCAT?

 

[ACSheldor]

The book (content or readings) is great, it's just some of the questions I have a problem with. The questions are either too tricky which is unrealistic or they are just flat out bad. Like for that question, the real MCAT would never have the last two choices. Because technically C could also be right. The MCAT is more straight forward or so I hear.

EK has a bad habit of having poorly worded questions like the one above. I've heard that real MCAT questions are more straight forward and better worded.

 

[RogueUnicorn]

The book (content or readings) is great, it's just some of the questions I have a problem with. The questions are either too tricky which is unrealistic or they are just flat out bad. Like for that question, the real MCAT would never have the last two choices. Because technically C could also be right. The MCAT is more straight forward or so I hear.

EK has a bad habit of having poorly worded questions like the one above. I've heard that real MCAT questions are more straight forward and better worded.

whoever told you this must be highly intelligent and probably very good looking as well

 

[ACSheldor]

Haha. If you have to say it then it's not probably true. Or at least that's what my wife says all the time.

 

[mjjdm1985]

Lol the more you say it the truer it gets. Thanks for all of the info!