Inner medullary collecting duct: urea recycling and final concentration
Collecting duct · UT-A1/A3 50-1200 mOsm ANP
The last segment, passing through the full 1200 mOsm/kg gradient. Under ADH it becomes permeable to urea as well as water, recycling urea into the inner medulla to sustain the gradient and delivering the final concentrated or dilute urine to the papilla.
Traced from the start
- Glomerular filtration barrier
- Proximal tubule: Na+/H+ exchange and bicarbonate reclamation
- Thick ascending limb of Henle (NKCC2)
- Early distal convoluted tubule: thiazide-sensitive NaCl cotransporter
- Inner medullary collecting duct: urea recycling and final concentration
Detail
- Reabsorbs
- Water via AQP2 across the entire medullary gradient (the final few percent of filtered water under maximal ADH); urea via apical UT-A1 and basolateral UT-A3
- Urea recycling
- ADH opens UT-A1/UT-A3 so urea concentrated by upstream water removal diffuses into the inner medulla, then into thin limbs and back; net urea excretion ~40-50% of the filtered load, less when ADH is high
- Osmolality
- Urine ranges from ~50 (no ADH) to ~1200 mOsm/kg (maximal ADH), specific gravity ~1.001 to 1.030; ADH also raises urine osmolality by raising medullary urea
- Free-water clearance
- C(H2O) = V - C(osm): positive during water diuresis, negative during antidiuresis
- Hormone
- ADH (V2) for water and urea; ANP from atrial stretch inhibits Na+ reabsorption here through cGMP-gated channels, opposing ADH and aldosterone (natriuresis)
- Diuretic
- None act here specifically; osmotic diuretics and vaptans reduce water abstraction
When it goes wrong
Very low urea (protein malnutrition, cirrhosis) or urea washout
Impaired maximal concentration: nocturia and mild polyuria despite intact ADH
Chronic obstruction or medullary sponge kidney
Collecting duct injury with concentrating defect, nephrocalcinosis, stones and distal RTA
High solute load (glucose, urea in catabolic states or after high-protein feeds, mannitol, IV saline)
Osmotic diuresis: polyuria with urine osmolality above 300 mOsm/kg but obligatory free-water loss causing hypernatraemia
Practise this structure
1 question in the bank tagged Water Balance.