Physiology · Renal, fluids and acid–base
The countercurrent mechanism, urea recycling and urine concentration !
- Mechanism
- Exam priority: high
The kidney salts its medulla so it can pull water out of urine when needed. That lets urine be up to four times more concentrated than blood.
Updated 2026-11-01
Also called: Countercurrent multiplier, Vasa recta
See it in 3D: Kidneys · Vessels inside the kidney
What it is
The loop of Henle, running in countercurrent, multiplies a small horizontal salt gradient into a medullary interstitium of ≈ 1200 mOsm/kg at the papillary tip. Recycled urea supplies almost half of that gradient, and the vasa recta preserve it without washing it out. With ADH, the collecting duct equilibrates with this medulla and produces concentrated urine.
How it works, step by step
- The thick limb moves NaCl into the interstitium without water, creating a ≈ 200 mOsm/kg difference between lumen and interstitium at each level.
- The water-permeable descending limb loses water to that interstitium and its fluid concentrates.
- Fluid moves on, carrying higher concentration toward the loop tip; the thick limb removes salt again, so the gradient multiplies with depth.
- In the inner medulla, the collecting duct with ADH lets water out and concentrates luminal urea.
- Urea exits through UT-A1 into the inner interstitium, partly enters the loop and returns to the collecting duct: it recycles.
- The vasa recta gain solute and lose water on the way down and do the reverse on the way up, removing reabsorbed water without carrying off the gradient.
- With high ADH, the collecting duct equilibrates with the medulla, so urine leaves almost as concentrated as the papilla.
The nephron: segments, transporters and diuretics
Na⁺ reabsorbed almost noneWater reabsorbed ≈ 15%
Thin descending limb of the loop of Henle
- What it does: Permeable to water, barely to solutes: water leaves into the hyperosmotic interstitium (≈ 15% of filtered water) and the fluid concentrates to ≈ 1200 mOsm/kg at the hairpin.
- Transporters: Aquaporin 1 (AQP1), always present: it does not depend on ADH.
- Drugs: Osmotic diuretics (3): keep water in the lumen here and in the proximal tubule.
- Disorders: Losing the medullary gradient (medullary washout) lowers the ability to concentrate urine.
- Glomerulus and arterioles
- Proximal tubule
- Thin descending limb
- Thin ascending limb
- Thick ascending limb
- Distal convoluted tubule
- Collecting tubule and duct
- Juxtaglomerular apparatus (macula densa)
- Cortex
- Medulla
- Interstitial osmolality (≈ 300 → 1200 mOsm/kg)
The nephron: segments, transporters and diuretics
Na⁺ reabsorbed small (passive)Water reabsorbed 0%
Thin ascending limb of the loop of Henle
- What it does: Impermeable to water; in the inner medulla NaCl leaves passively and the fluid starts to dilute. With the thick limb and urea it builds the medullary gradient (countercurrent multiplier).
- Transporters: Cl⁻ channels (ClC-K1) and a paracellular path for Na⁺; passive urea transport.
- Glomerulus and arterioles
- Proximal tubule
- Thin descending limb
- Thin ascending limb
- Thick ascending limb
- Distal convoluted tubule
- Collecting tubule and duct
- Juxtaglomerular apparatus (macula densa)
- Cortex
- Medulla
- Interstitial osmolality (≈ 300 → 1200 mOsm/kg)
Approximate osmolalities
| Parameter | Value | Note |
|---|---|---|
| Plasma | ≈ 285–295 mOsm/kg | |
| Cortical interstitium | ≈ 300 mOsm/kg | |
| Papillary tip interstitium | ≈ 1200 mOsm/kg | can reach ≈ 1400 |
| Urine, maximal concentration | ≈ 1200 mOsm/kg | |
| Urine, maximal dilution | ≈ 50 mOsm/kg | |
| Obligatory minimum urine output | ≈ 0.5 L/day | with ≈ 600 mOsm/day of solute |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Who does what
| Element | Role | If it fails |
|---|---|---|
| Thick ascending limb | Engine: removes NaCl without water | Loop diuretics: urine can be neither well concentrated nor diluted |
| Descending limb | Equilibrates by losing water | — |
| Collecting duct + ADH | Lets water out; concentrates urea | Diabetes insipidus: large volumes of dilute urine |
| Urea (UT-A1) | ≈ 40–50% of the inner gradient | Low-protein diet: less concentration |
| Vasa recta | Exchange: preserve the gradient | High medullary flow: medullary washout |
Clinical notes
- Central or nephrogenic diabetes insipidus: without ADH effect, urine stays dilute (< 300 mOsm/kg) even as the patient dehydrates.
- Long-standing primary polydipsia washes out the medulla: even with ADH, urine does not reach maximal concentration at first.
- Older adults concentrate urine less well, so they are at higher risk of hypernatremia without access to water.
Test yourself
Think of the answer, then tap the question to see it.
Why does a loop diuretic reduce the ability to concentrate urine?
It blocks NKCC2 in the thick limb, the engine that moves salt into the interstitium. Without that medullary gradient, ADH has nowhere to draw water from the collecting duct.
What role does urea play in concentrating urine?
ADH opens UT-A1 in the inner medullary collecting duct; concentrated urea leaves into the interstitium and supplies almost half its osmolality, letting more water be withdrawn.
Why don't the vasa recta wash out the medullary gradient?
They are U-shaped with slow flow: they gain solute going down and return it going up (countercurrent exchange), while carrying away reabsorbed water.
Why is there an obligatory minimum urine output of ≈ 0.5 L/day?
About 600 mOsm of solute must be excreted daily and urine cannot exceed ≈ 1200 mOsm/kg; dividing gives at least ≈ 0.5 L.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Transport in the loop of Henle
- ADH, thirst and water balance
- Urine dilution, free water and osmotic diuresis
- Functional anatomy of the nephron
- Distal tubule and collecting duct
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 25.6 Tubular Reabsorption (OpenStax (Rice University), 2022); 26.2 Water Balance (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 29). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.