Dilute and Concentrated Urine Formation

Dilute Urine (ADH low/absent)

  • Trigger ➜ over-hydration, plasma osmolarity ↓
  • Thick ascending limb (TAL)
    • Impermeable to water; NKCC pumps Na\^+, K\^+, 2 Cl\^- out
    • Tubular fluid osmolarity falls to ≈ 150 mOsm vs IF 300
  • Descending limb
    • Impermeable to salts, permeable to water
    • Water exits → tubular fluid concentrates until iso-osmotic with IF
  • Late distal convoluted tubule (DCT) & collecting duct (CD)
    • Principal cells lack aquaporins → water cannot follow Na\^+
    • Na\^+ actively reabsorbed; tubular fluid osmolarity drops (≈ 100→50 mOsm)
    • Few urea transporters only near papillary end; limited urea recycling
  • Result ➜ large volume, hypo-osmotic urine; blood volume ↓

Concentrated Urine (ADH high)

  • Trigger ➜ dehydration, plasma osmolarity ↑
  • Common loop actions remain (salt pump in TAL, water equilibration in descending limb)
  • ADH effects on late DCT & CD
    • Inserts aquaporin-2 → water reabsorption ↑ dramatically
    • Up-regulates urea transporters along CD
  • Consequences
    • Water exit concentrates tubular urea
    • Steeper urea gradient drives urea out of CD into medulla → enters thin limbs → recycles
    • Urea + NaCl maintain steep medullary gradient (up to ≈ 1200 mOsm)
  • Result ➜ small volume, hyper-osmotic urine; plasma osmolarity ↓

Segment Functions (key points)

  • TAL: “Pump” – sets gradient via active NaCl transport, no water flow
  • Descending limb: “Equilibrate” – passive water loss to match IF
  • Collecting duct: variable water & urea handling; ADH-regulated
  • Vasa recta: counter-current exchanger preserves medullary gradient

Urea Recycling Essentials

  • ADH ↑ CD urea transporters → more urea enters medulla
  • Urea diffuses into thin limbs, returns to TAL/DCT, re-enters CD
  • Reinforces medullary osmotic gradient; critical for maximal urine concentration

Diuretics (clinical tie-in)

  • Caffeine ➜ inhibits Na\^+ reabsorption in DCT/CD → water retained in lumen
  • Loop diuretics (e.g., Lasix) ➜ block NKCC in TAL → medullary gradient ↓ → water loss ↑
  • Alcohol ➜ suppresses ADH release → CD water permeability ↓ → diuresis