Nephron confusion: if the descending limb loses water and the ascending limb loses salt, who exactly concentrates the urine?
Ananya Reddy11th Sample
15 Sept 2026 Β· 1 min read
Doing excretory products in class 11 and I made a colour-coded flowchart (green for reabsorption, red for secretion) but now I'm stuck on the loop of Henle and my flowchart doesn't help.
What I understood: filtrate goes PCT, then loop of Henle (descending then ascending limb), then DCT, then collecting duct. In the PCT most of the useful stuff like glucose and amino acids gets reabsorbed.
What I don't understand:
- NCERT says the descending limb is permeable to water but not to electrolytes, so the filtrate gets concentrated as it goes down. Then the ascending limb is impermeable to water but lets electrolytes go out, so the filtrate gets diluted going up. If it ends up diluted anyway, why is the loop of Henle even useful?
- Where does ADH act, and where does aldosterone act? I keep mixing them up. I wrote both as "increase water reabsorption".
- Why is the medulla saltier as you go deeper (NCERT says 300 mOsmol/L in the cortex going up to about 1200 in the inner medulla)? Who is putting that salt there?
I tried watching a video but it said "countercurrent" so many times that I stopped following. Can someone explain it in simple words?
11 answers & replies
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[+][β]Diya Menon11thSampleΒ· 1mo ago
ahh I had the same confusion!! so think of the loop as a salt pump creating a salty "environment" around the tube. Ascending limb pushes salt OUT into the tissue around it (that's why the filtrate inside becomes dilute). That salty tissue then pulls water out of the descending limb by osmosis. So the loop isn't about the fluid inside being concentrated at the end, it's about making the medulla salty so the collecting duct can later pull water out.
[+][β]Ananya Reddy11thSampleΒ· 1mo agoAuthor
ohhh okay. so the dilution in the ascending limb is just a side effect of dumping salt into the medulla?
[+][β]Diya Menon11thSampleΒ· 1mo ago
yes exactly! the dilute filtrate is the "leftover" of salt being dumped out. omg I should've said it that way first π
[+][β]Meera Patel11thSampleΒ· 1mo ago
Let me answer 2 and 3 properly.
Who adds the salt? The ascending limb moves NaCl into the interstitium. The collecting duct also lets a little urea move into the medulla. Together NaCl and urea build the gradient from about 300 mOsmol/L at the cortex to about 1200 mOsmol/L in the inner medulla. The vasa recta runs parallel to the loop in the opposite direction, which is the "countercurrent" part: it takes away water and returns salt without washing out the gradient.
ADH vs aldosterone. ADH (from the neurohypophysis) acts mainly on the DCT and collecting duct. It makes them more permeable to water, so water leaves the filtrate and returns to the blood, and the urine is concentrated. Aldosterone (from the adrenal cortex) is part of the renin-angiotensin mechanism. It acts on the DCT to reabsorb NaβΊ and water, and it raises blood pressure. So both end up conserving water but they work at different points through different triggers: ADH is triggered by high osmolarity or low volume, aldosterone through the juxtaglomerular cells releasing renin.
[+][β]Ananya Reddy11thSampleΒ· 1mo agoAuthor
Thank you everyone, this is the first time the loop made sense. Redoing my flowchart tonight with "salt out" and "water out" arrows in different colours (yes, I know)
[+][β]Yash Tiwari11thSampleΒ· 1mo ago
bhai mai to ye soch ke baitha tha ki ascending limb me pani nikalta hai kyunki upar jaate jaate urine patla ho raha hai. toh ascending limb water absorb karta hai na? π
[+][β]Kavya NaikDropperSampleΒ· 1mo ago
Not quite Yash, the opposite. The ascending limb is impermeable to water. The filtrate gets dilute because salt leaves, but water stays inside. Descending limb is where water leaves. That "descending = water out, ascending = salt out" line is worth writing at the top of your page.
[+][β]Yash Tiwari11thSampleΒ· 1mo ago
ohh samajh gaya, galat soch rha tha. thanks didi
[+][β]Lakshmi TripathiDropperSampleΒ· 1mo ago
Small addition for your flowchart. The PCT does the bulk of the reabsorption. Nearly all the glucose and amino acids, and about 70-80 percent of the electrolytes and water, get back in the PCT. So by the time the filtrate reaches the loop of Henle it is already much smaller in volume. The loop then does the fine tuning for concentrating, and the DCT and collecting duct do conditional reabsorption depending on hormones.
[+][β]Tanvi Deshmukh12thSampleΒ· 1mo ago
Make a three-column table!!! Columns: segment, permeable to, what happens. I do one for every organ system. PCT / descending / ascending / DCT / collecting duct, and put the hormone next to the segment it acts on. Mine saved me in a class test, it's literally one glance π
[+][β]Nisha Verma12thSampleΒ· 1mo ago
From a class 12 who revised this for boards: the ANF (atrial natriuretic factor) is the one students forget. When blood pressure rises, the atria release it and it dilates blood vessels and lowers the pressure, so it opposes renin-angiotensin. Examiners love "which hormone opposes the RAAS". Ask me how I know.
