Countercurrent multiplier: how the kidney concentrates urine
No pump in the kidney moves water, yet urine can be several times as concentrated as plasma. The countercurrent multiplier, urea recycling and vasa recta build the medullary gradient that ADH lets the duct use.
Chapters (10)
Explain how the loop of Henle's countercurrent multiplier, urea recycling and the vasa recta create the medullary gradient used to concentrate urine.
The short version
- The thick ascending limb pumps salt without water, a single effect of about 200 mOsm/kg, which the loop multiplies toward about 1200 mOsm/kg at the tip of the medulla.
- The thin descending limb is permeable to water through aquaporin 1, so its fluid becomes more concentrated at each step down toward the hairpin turn.
- Urea recycling deepens the inner medulla, and the vasa recta, a countercurrent exchanger, preserves the gradient instead of washing it out.
- ADH inserts aquaporin 2 into the collecting duct membrane, so water leaves into the salty medulla and urine is concentrated.
- Loop diuretics block NKCC2, so the single effect shrinks, the gradient is blunted and urine stays closer to plasma osmolality.
Concentrated urine with no water pump
A man skips water overnight. By morning his plasma is 285 mOsm/kg and his urine is 1050 mOsm/kg. The urine is more than three times as concentrated as his blood, and the kidney made it with no pump that moves water.
The explanation is the countercurrent multiplier in the loop of Henle. Together with urea recycling and the vasa recta, it builds the medullary gradient that concentrates urine.
The mental model: the medullary gradient
The proximal tubule reabsorbs about two thirds of the filtrate, with water following solute. Fluid therefore reaches the loop of Henle still at about 300 mOsm/kg, the same as plasma.
The medulla, though, is saltier the deeper you go: about 300 mOsm/kg in the cortex, rising to about 1200 mOsm/kg at the tip. The loop of Henle builds that gradient, and the collecting duct then uses it.[1]
| Region | Osmolality (mOsm/kg) |
|---|---|
| Cortex | about 300 |
| Outer medulla | about 800 |
| Inner medullary tip | up to about 1200 |
The thick ascending limb and the single effect
The key trick is the thick ascending limb. Its NKCC2 transporter moves sodium, potassium and two chlorides out of the tubule. The limb is also waterproof, so water cannot follow.[3]
Salt leaves and water stays: the tubular fluid becomes dilute and the interstitium becomes salty. That difference is the single effect, about 200 mOsm/kg at any one level.
The descending limb behaves differently. It is permeable to water through aquaporin 1, so water leaves until the fluid matches the salty interstitium. The result is that fluid becomes more concentrated at each step down toward the hairpin turn.[1][2]
Why a small single effect multiplies
A small single effect, repeated along the flow, stacks up. That is what makes the loop a countercurrent multiplier.[3]
- Both limbs start at 300 mOsm/kg.
- The ascending limb pumps until it is 200 lower than its surroundings: the interstitium sits at 400 and the ascending fluid at 200.
- The descending limb equilibrates up to 400.
- New fluid pushes that 400 around the bend, the ascending limb pumps again, and the floor rises.
- Repeat, and the tip climbs toward 1200.
Mechanism summary: pump, equilibrate, flow, repeat. The worked cycle is a simplified teaching model, and the figures are approximate.[3]
Urea recycling and the vasa recta
In the inner medullary collecting duct, ADH makes the duct more permeable to urea. Urea leaves the duct into the deep medulla and adds to its osmolality. Some of it re-enters the thin limbs and recycles.[1][2]
The vasa recta are capillaries that run as hairpins beside the loops. Blood going down gains salt and loses water, and blood coming up does the reverse. The exchange keeps the gradient in place instead of washing it out.[1][2]
ADH, aquaporin 2 and loop diuretics
ADH then inserts aquaporin 2 into the collecting duct membrane, and water leaves into the salty medulla. That is the concentrated urine from the start of the lesson.
Loop diuretics block NKCC2, so the single effect shrinks, the gradient is blunted, and urine stays closer to plasma osmolality.
Practice question
Original · USMLE styleA 62-year-old man with heart failure is started on oral furosemide. Three days later, as part of a research protocol, he abstains from fluids for 12 hours. Plasma osmolality is 292 mOsm/kg and urine osmolality is 340 mOsm/kg. Plasma antidiuretic hormone concentration is high. Two months earlier, before furosemide, the same protocol produced a urine osmolality of 880 mOsm/kg.
Common questions
What is the countercurrent multiplier?
It is the loop of Henle's mechanism for building the medullary gradient. The thick ascending limb pumps salt without water, a single effect of about 200 mOsm/kg, and the flow around the hairpin repeats it so the tip climbs toward about 1200 mOsm/kg.
What is the single effect in the loop of Henle?
It is the difference created when the waterproof thick ascending limb moves sodium, potassium and chloride out through NKCC2 without water, making the tubular fluid dilute and the interstitium salty. It is about 200 mOsm/kg at any one level.
What do the vasa recta do in urine concentration?
They are hairpin capillaries beside the loops. Descending blood gains salt and loses water and ascending blood does the reverse, so the exchange keeps the medullary gradient in place instead of washing it out.
Why do loop diuretics impair urine concentration?
They block NKCC2 in the thick ascending limb. The single effect shrinks, the gradient is blunted and urine stays closer to plasma osmolality.
Transcript
Read the full narration, by chapter660 words
0:00A concentrated urine with no water pump
A man skips water overnight. By morning his plasma is 285 milliosmoles per kilogram, and his urine is 1050. The urine is more than three times as concentrated as his blood, and the kidney made it with no pump that moves water. Today: explain how the loop of Henle, urea and the vasa recta build the medullary gradient that concentrates urine.
0:26The mental model: the medullary gradient
First, the mental model. The proximal tubule reabsorbs about two thirds of the filtrate, with water following solute. So fluid reaches the loop of Henle still about 300 milliosmoles per kilogram, the same as plasma. The medulla, though, is saltier the deeper you go: about 300 in the cortex, rising to about 1200 at the tip. The loop of Henle builds that gradient, and the collecting duct then uses it.
0:54Thick ascending limb: the single effect
Start with the key trick: the thick ascending limb. Its NKCC two transporter moves sodium, potassium and two chlorides out of the tubule. And this limb is waterproof, so water cannot follow. Salt leaves, water stays: the tubular fluid becomes dilute, and the interstitium becomes salty. That difference is the single effect, about 200 milliosmoles per kilogram at any one level.
1:23Descending limb: water leaves
Next, the descending limb. It is permeable to water through aquaporin one, so water leaves until the fluid matches the salty interstitium. The result: fluid gets more concentrated at each step down toward the hairpin turn.
1:39Why a small single effect multiplies
Here is why it multiplies. A small single effect, repeated along the flow, stacks up. Watch one cycle. Both limbs start at 300. The ascending limb pumps until it is 200 lower than its surroundings, so the interstitium sits at 400 and the ascending fluid at 200. The descending limb equilibrates up to 400. New fluid pushes that 400 around the bend, the ascending limb pumps again, and the floor rises. Repeat, and the tip climbs toward 1200. Mechanism summary: pump, equilibrate, flow, repeat.
2:19Urea recycling
Now urea. In the inner medullary collecting duct, ADH makes the duct more permeable to urea. Urea leaves the duct into the deep medulla and adds to its osmolality. Some of it re-enters the thin limbs and recycles.
2:35Vasa recta: keeping the gradient
Last, the vasa recta. These capillaries run as hairpins beside the loops. Blood going down gains salt and loses water, and blood coming up does the reverse. The exchange keeps the gradient in place instead of washing it out. ADH then inserts aquaporin two into the
2:51ADH, aquaporin-2 and the concentrated urine
collecting duct membrane, and water leaves into the salty medulla. That is the concentrated urine from the start. And loop diuretics block NKCC two, so the single effect shrinks, the gradient is blunted, and urine stays closer to plasma osmolality.
3:12Practice question
Your turn. A 62-year-old man with heart failure started furosemide 3 days ago. After 12 hours without fluids his urine osmolality is 340, plasma is 292, and ADH is high. Two months ago, off the drug, the same test gave 880. Which best explains his urine? A, higher water permeability of the thick ascending limb. B, more urea recycling. C, absent ADH secretion. D, blocked sodium chloride reabsorption in the thick ascending limb. E, increased aquaporin two insertion in the collecting duct. Pause here and decide. A is out: that limb is waterproof, and furosemide does not change that. B is out: more urea recycling would raise the gradient, not lower it. C is out: his ADH is high, so the signal is present. E is out: more aquaporin two would pull more water out and raise urine osmolality, the opposite of what we see. So the answer is D: furosemide blocks NKCC two, the single effect shrinks, and urine cannot be fully concentrated. Three things to keep.
4:33High-yield recap and next lesson
One: the thick ascending limb pumps salt without water, a single effect of about 200, which the loop multiplies toward 1200. Two: urea recycling deepens the inner medulla, and the vasa recta preserves it. Three: loop diuretics blunt the gradient and the kidney loses its power to concentrate. Next up: the distal nephron, aldosterone and ADH.
Sources
Numbers in the text, e.g. [1], point here- The counter-current mechanisms in the kidney. Deranged Physiology. derangedphysiology.com/main/cicm-primary-exam/required-reading/renal-system/Chapter%20006/counter-current-mechanisms-kidney
- Dantzler WH et al. Urine concentrating mechanism in the inner medulla of the mammalian kidney. Clin J Am Soc Nephrol 2014. PMC4186519. pmc.ncbi.nlm.nih.gov/articles/PMC4186519
- Sands JM, Kokko JP. Current concepts of the countercurrent multiplication system. Kidney Int Suppl 1996. pubmed.ncbi.nlm.nih.gov/8941928/
The 300 / 800 / 1200 mOsm/kg gradient and the single effect of about 200 mOsm/kg are approximate teaching figures; the worked cycle is a simplified classic model, and the bar chart after the first cycle is schematic.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustration (nephron plate in the video): Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized). All other diagrams are original (code-drawn). Example lab values are illustrative.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-027.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01GFR and Starling forces at the glomerulus5:25
- EP 02Afferent vs efferent arteriole: GFR, RPF and filtration fraction5:16
- EP 03Renal clearance: inulin, creatinine and PAH5:20
- EP 04Glucose transport maximum, splay and SGLT24:55
- EP 05Countercurrent multiplier: how the kidney concentrates urine5:02
- EP 06Aldosterone and ADH in the distal nephron5:20
- EP 07Diuretics by nephron segment: loop, thiazide and more5:53
- EP 08RAAS explained: renin, angiotensin II and aldosterone5:23