Achiever SNS #2 Q30

[lifeisgood]

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Why are the lower portions of loops of Henle and the collecting ducts selectively permeable to NaCl, urea, and/or water?



A. To increase the osmolarity of interstitial fluid in the cortex and help prevent water loss from the nephronic filtrate
B. To decrease the osmolarity of interstitial fluid in the cortex and help promote water loss from the nephronic filtrate
C. To increase the osmolarity of interstitial fluid in the inner medulla and help prevent water loss from the nephronic filtrate
D. To decrease the osmolarity of interstitial fluid in the inner medulla and help promote water loss from the nephronic filtrate
E. No specific reason can be deduced from the said phenomenon


Answer is C and I am trying to figure out why. It seems counterintuitive to me. The entire reason of the counter-current-multiplier system is to remove H20 from the filtrate. And increasing the osmolarity of the adrenal medulla does just that, it makes the medulla HYPERtonic to the filtrate, thus causing it to lose water. WTF?

 

[lifeisgood]

Why no love? Amsie, come on help me out. You read all 3 Schaums!

 

[lifeisgood]

Since nobody wanted to help, I found the answer. I was right.

According to Schaum's (page 240 - diagram & paragraph above) as the filtrate desecnds down the loop of Henle water rushes out. On the way up, Na+ and Cl- ions rush out (while the tubule is impermeable to water) to maintain the hyperosmolarity of the medulla.

Thus, the final answer is that the increase in the osmolarity of interstitial fluid in the inner medulla helps promote water loss from the nephronic filtrate.

So none of the answers are correct. w00t!

If I am wrong please let me know.