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[Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal)

# Urine dilution, free water and osmotic diuresis

- Measurement
- Exam priority: medium

The kidney can excrete water without salt (dilute urine) or hold on to it; free-water clearance shows which way it is working.

Updated 2026-11-01

Also called: Free-water clearance, Diluting segment, Polyuria

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- Diluting segment: thick ascending limb (NKCC2) and distal tubule (NCC), both impermeable to water.
- Without ADH, minimum urine ≈50 mOsm/kg; with ADH, maximum ≈1200 mOsm/kg in adults.
- Positive CH₂O = urine more dilute than plasma (water excreted); negative = concentrated urine (water retained).
- The ability to excrete water depends on solute output: with very low solute intake (beer only, or tea and toast), water is retained.
- Thiazides block the distal tubule and impair dilution: a classic cause of hyponatraemia. Loop diuretics impair both dilution and concentration.
- Osmotic diuresis (glucose, mannitol, urea): copious urine near isosmotic, with loss of Na⁺ and water.

## What it is

To excrete excess water, the kidney separates salt from water in the thick ascending limb and the distal tubule, which reabsorb NaCl but are water-tight: fluid leaves them dilute (≈100 mOsm/kg). Without ADH, the collecting duct lets that dilute urine through (down to ≈50 mOsm/kg). Free-water clearance (CH₂O) tells how much “pure” water is being excreted (+) or retained (−). In osmotic diuresis, an unreabsorbed solute drags water and sodium out with it.

### The nephron: segments, transporters and diuretics

**Na⁺ reabsorbed** ≈ 25% **Water reabsorbed** 0%

#### Thick ascending limb of the loop of Henle

- **What it does:** Reabsorbs ≈ 25% of filtered Na⁺ but is impermeable to water: the diluting segment (fluid leaves at ≈ 100 mOsm/kg) and the engine of the medullary gradient. The lumen-positive potential drives paracellular Ca²⁺ and Mg²⁺ reabsorption.
- **Transporters:** Apical NKCC2 (Na⁺-K⁺-2Cl⁻); ROMK recycles K⁺ to the lumen; basolateral ClC-Kb with barttin; claudins 16 and 19 (paracellular path); Ca²⁺-sensing receptor.
- **Hormones:** ADH (V2) increases NaCl reabsorption; PTH increases Ca²⁺ reabsorption.
- **Drugs:** Loop diuretics (4): block NKCC2; Na⁺, K⁺, Ca²⁺ and Mg²⁺ are lost (hypokalemia, metabolic alkalosis, hypercalciuria).
- **Disorders:** Bartter syndrome (like a loop diuretic: hypokalemic alkalosis, hypercalciuria, high renin and aldosterone).

- 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)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

### The nephron: segments, transporters and diuretics

**Na⁺ reabsorbed** ≈ 2–5% **Water reabsorbed** variable (ADH)

#### Collecting tubule and duct

- **What it does:** Fine tuning: reabsorbs ≈ 2–5% of Na⁺, secretes K⁺ and H⁺ and reabsorbs water as ADH allows (urine from ≈ 50 to ≈ 1200 mOsm/kg). Principal cells handle Na⁺, K⁺ and water; intercalated cells handle acid–base.
- **Transporters:** Principal: ENaC, ROMK and BK (K⁺ secretion; BK is flow-driven), apical AQP2 (ADH) and basolateral AQP3/4. α-intercalated: H⁺-ATPase and H⁺/K⁺-ATPase (secrete acid). β-intercalated: pendrin (secrete HCO₃⁻). Inner medulla: urea transporters (ADH).
- **Hormones:** Aldosterone (more ENaC, ROMK, Na⁺/K⁺-ATPase and H⁺-ATPase); ADH via the V2 receptor → cAMP → AQP2 insertion; atrial natriuretic peptide curbs Na⁺ reabsorption.
- **Drugs:** ENaC blockers (6) and mineralocorticoid receptor antagonists (7): K⁺-sparing (risk of hyperkalemia). V2 receptor antagonists (8, vaptans): aquaretics.
- **Disorders:** Liddle syndrome (overactive ENaC: hypertension, hypokalemia, low renin and aldosterone); nephrogenic diabetes insipidus (V2 or AQP2, lithium); type 1 distal renal tubular acidosis (H⁺ secretion fails: urine pH > 5.5, hypokalemia, stones) and type 4 (hypoaldosteronism: hyperkalemia); SIADH (excess ADH).

- 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)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

## Equations

**Osmolar clearance**Cosm = (Uosm × V) / Posm

**Free-water clearance**CH2O = V − Cosm

**V**
urine flow

**C osm**
osmolar clearance

If U_{osm} 0.

**Urine volume from solute load**V ≈ solute excreted / Uosm

600 mOsm/day ÷ 1200 mOsm/kg ≈ 0.5 L/day obligatory minimum; ÷ 50 ≈ 12 L/day maximum.

## Approximate figures

| Parameter | Value | Note |
| --- | --- | --- |
| Minimum urine osmolality | ≈50 mOsm/kg | |
| Maximum urine osmolality | ≈1200 mOsm/kg | lower in infants and older adults |
| Fluid leaving the ascending limb | ≈100 mOsm/kg | always hypotonic |
| Daily solute excretion | ≈600–900 mOsm | depends on diet (salt, protein) |
| Obligatory minimum urine volume | ≈0.5 L/day | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Polyuria: water or solute

| Type | Urine osmolality | CH₂O | Examples |
| --- | --- | --- | --- |
| [Water diuresis](https://durgical.com/en/conditions/hypernatraemia/) | Low (
- In SIADH urine stays concentrated despite hyponatraemia: CH₂O is negative; water restriction works, and isotonic saline can worsen it if the urine is more concentrated than the infused fluid.
- In diabetes insipidus free water is lost: dilute polyuria and hypernatraemia if the person cannot drink.
- Older adults eating little but drinking a lot (or drinking a lot of beer) excrete little solute: they retain water even with low ADH.

## Memory aid

“Urine needs passengers”: every litre of water leaves with solute; without solute, the water stays.

## Test yourself

Think of the answer, then tap the question to see it.

## Someone passes 2 mL/min of urine at 600 mOsm/kg with plasma at 300. What is CH₂O and what does it mean?

Cosm = 600 × 2 / 300 = 4 mL/min; CH₂O = 2 − 4 = −2 mL/min. It is negative: the kidney is retaining free water (ADH-concentrated urine).

## Why do thiazides cause hyponatraemia more often than loop diuretics?

They block the distal tubule (diluting segment) but spare the loop and the medullary gradient: the kidney cannot dilute well yet can still concentrate under ADH, so it retains water.

## Why can a very low-solute diet cause hyponatraemia even with ADH suppressed?

Maximum urine volume is solute ÷ minimum Uosm. With, say, 250 mOsm/day and 50 mOsm/kg, only ≈5 L/day can be excreted; drinking more makes water accumulate.

## How does urine osmolality separate polyuria from osmotic diuresis versus diabetes insipidus?

In osmotic diuresis urine is near or above 300 mOsm/kg (it carries a lot of solute); in diabetes insipidus it is very dilute, below ≈150 mOsm/kg.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/urine-dilution-free-water-and-osmotic-diuresis/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics

- [The countercurrent mechanism, urea recycling and urine concentration](https://durgical.com/en/physiology/the-countercurrent-mechanism-urea-recycling-and-urine-concentration/)
- [ADH, thirst and water balance](https://durgical.com/en/physiology/adh-thirst-and-water-balance/)
- [Transport in the loop of Henle](https://durgical.com/en/physiology/transport-in-the-loop-of-henle/)
- [Where and how diuretics act](https://durgical.com/en/physiology/where-and-how-diuretics-act/)
- [Glucose transport maximum and renal threshold](https://durgical.com/en/physiology/glucose-transport-maximum-and-renal-threshold/)
- [Fluid shifts between compartments](https://durgical.com/en/physiology/fluid-shifts-between-compartments/)

## Related conditions

- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Syndrome of inappropriate antidiuretic hormone secretion (SIADH)](https://durgical.com/en/conditions/syndrome-of-inappropriate-antidiuretic-hormone-secretion/)
- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)

## In Biochemistry

- [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/)
- [Urea](https://durgical.com/en/biochemistry/molecules/urea/)

[← All of physiology](https://durgical.com/en/physiology/)

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: [26.2 Water Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-2-water-balance) (OpenStax (Rice University), 2022); [25.9 Regulation of Fluid Volume and Composition](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-9-regulation-of-fluid-volume-and-composition) (OpenStax (Rice University), 2022); [Osmolality Tests: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/osmolality-tests/) (MedlinePlus, US National Library of Medicine, 2024); 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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/urine-dilution-free-water-and-osmotic-diuresis/