Renal Physiology · Loop of Henle

Counter-Current
Mechanism

How the kidney creates a concentration gradient to produce concentrated urine

Speed 1×
Key Values
300 mOsm/kg
142 Na⁺ mmol/L
300 Urine mOsm
Descending Limb — H₂O permeable
Ascending Limb — Na⁺/K⁺/Cl⁻ pump
Vasa Recta — countercurrent exchange
Collecting Duct — ADH-controlled
NaCl / Solute particle
H₂O molecule
Current Phase
Proximal Tubule
Isotonic reabsorption — 65% of filtered Na⁺ and water are reabsorbed here, maintaining osmolality around 300 mOsm/kg.
Medullary Osmotic Gradient
300 mOsm
600 mOsm
900 mOsm
1200 mOsm
Key Concept
Why Counter-Current?
Fluid flows DOWN in one tube and UP in an adjacent tube. This arrangement amplifies small concentration differences into a large gradient across the medulla — impossible with parallel flow.
Clinical Relevance
ADH / Vasopressin
Without ADH, collecting duct remains water-impermeable → dilute urine (diabetes insipidus). With ADH, aquaporin-2 channels open → concentrated urine up to 1200 mOsm/kg.