Step by Step
1
The descending limb — water leaves
The descending limb of the loop of Henle is permeable to water but impermeable to solutes — as fluid flows through, water leaves (drawn out by the surrounding hypertonic medulla), and the remaining tubular fluid becomes progressively more concentrated.
2
The ascending limb — salt is pumped out
The thick ascending limb is impermeable to water but actively pumps NaCl out into the surrounding medullary interstitium — this is what makes the medulla increasingly hypertonic, reaching up to 1200 mOsm at the papilla (compared to only about 300 mOsm in the cortex).
3
The collecting duct — where ADH makes the real difference
As the collecting duct descends back through this same hypertonic gradient, its behavior depends entirely on ADH. Without ADH, water stays trapped in the duct, producing dilute urine. With ADH, aquaporin channels are inserted, allowing the surrounding osmotic gradient to draw water out — producing concentrated urine, up to 1200 mOsm.
4
The vasa recta — preserving the gradient
These specialized capillaries loop alongside the loop of Henle in a countercurrent arrangement, allowing blood flow through the medulla without washing away the carefully built-up hypertonic gradient.
Applied Walkthrough
1
As tubular fluid flows down the descending limb of the loop of Henle, water is drawn out by the surrounding hypertonic medulla, concentrating the fluid remaining inside the tubule.
2
As this now-concentrated fluid flows back up the thick ascending limb, NaCl is actively pumped out (while water stays put, since this segment is impermeable to it) — reinforcing the hypertonic gradient in the surrounding medullary tissue.
3
This process builds a striking osmotic gradient — up to 1200 mOsm deep in the medulla near the papilla, compared to only about 300 mOsm in the cortex.
4
When this same fluid later passes through the collecting duct (which runs back down through this same gradient), the presence or absence of ADH determines the outcome: without ADH, the duct stays impermeable to water and dilute urine results; with ADH, aquaporin channels open up and the surrounding osmotic gradient pulls water out, concentrating the urine — all made possible by the gradient the loop of Henle worked to establish.
Exam Application
Exams test whether you understand how the descending and ascending limbs work together (in opposite ways) to build the medullary concentration gradient, and whether you can explain how the collecting duct uses that gradient — but only when ADH is present — to concentrate urine.
⚠ Common Trap
The most common trap is assuming the collecting duct alone concentrates urine — it can only do so because of the hypertonic gradient the loop of Henle has already built up; without that gradient (and without ADH signaling the duct to become water-permeable), no concentration would occur.
✓ Quick Self-Check
1. What is permeable to in the descending limb, and what happens as a result?
Water (not solutes); water leaves, concentrating the tubular fluid.
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2. What is permeable to in the ascending limb, and what happens as a result?
Solutes (not water); NaCl is actively pumped out, building the medullary hypertonic gradient.
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3. What is the osmotic gradient range from the cortex to the deep medulla?
About 300 mOsm in the cortex to up to 1200 mOsm at the papilla.
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4. What determines whether the collecting duct produces dilute or concentrated urine?
The presence of ADH — without it, urine stays dilute; with it, aquaporins allow water to be drawn out, concentrating the urine.
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5. What is the role of the vasa recta?
To allow blood flow through the medulla in a countercurrent arrangement, without washing away the hypertonic gradient.
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