Countercurrent multiplier and the medullary osmotic gradient
Loop of Henle · 300 to 1200 Single effect Urea ~50%
The loop's U-shape turns the thick ascending limb's modest single effect (~200 mOsm/kg across its wall) into a corticomedullary gradient of 300 to ~1200 mOsm/kg, which the collecting duct later uses to concentrate urine. Urea recycled from the inner medullary collecting duct supplies about half of the deepest interstitial osmolality.
Traced from the start
- Glomerular filtration barrier
- Proximal tubule: Na+/H+ exchange and bicarbonate reclamation
- Countercurrent multiplier and the medullary osmotic gradient
Detail
- Single effect
- At any level the water-impermeable ascending limb pumps NaCl out, making the interstitium and the adjacent descending limb ~200 mOsm/kg more concentrated than ascending fluid
- Multiplication
- Flow in opposite directions stacks the single effects longitudinally: descending fluid keeps meeting ever more concentrated interstitium, so the hairpin reaches ~1200 mOsm/kg (cortex 300, outer medulla ~600, papilla ~1200)
- Composition
- Outer medulla is mostly NaCl; inner medulla is ~50% urea recycled via ADH-dependent UT-A1/UT-A3 from the collecting duct into the interstitium and thin limbs
- Preservation
- Vasa recta act as a countercurrent exchanger, and slow medullary blood flow (~5-10% of renal blood flow) prevents washout
- Output
- Urine from ~50 mOsm/kg (no ADH) to ~1200 mOsm/kg (maximal ADH); minimum obligatory urine volume ~500 mL/day to clear ~600 mOsm of daily solute
When it goes wrong
Loop diuretics (NKCC2 blocked, single effect abolished)
Gradient washes out within hours: urine can be neither concentrated nor diluted (isosthenuria ~300 mOsm/kg)
Low-protein diet or malnutrition (low urea)
Reduced inner medullary tonicity and impaired maximal urine concentration
Sickle cell trait or disease, analgesic nephropathy, chronic obstruction or tubulointerstitial disease
Loss of concentrating ability presenting as polyuria and nocturia before GFR falls; papillary necrosis in sickle cell and analgesic nephropathy
Practise this structure
2 questions in the bank tagged Counter Current Mechanism.
A 32-year-old woman has passed large volumes of pale urine for four months and drinks about eight litres of water each day, having decided this would improve he…
Foundations · middle twist
A 7-year-old boy has passed large volumes of urine and craved salty food since infancy, and he is small for his age. His blood pressure is 92/56 mmHg and he has…
Foundations · middle twist