Aquaporin-2 mediated water reabsorption
End hormone or mediator · AQP2 Free water Medullary gradient
Apical aquaporin-2 lets water follow the medullary osmotic gradient out of the collecting duct, returning solute-free water to the circulation and concentrating the urine.
Traced from the start
- Supraoptic and paraventricular magnocellular neurones
- Posterior pituitary — vasopressin release
- Collecting duct principal cell (V2 receptor)
- Aquaporin-2 mediated water reabsorption
Detail
- Output
- With maximal ADH the urine falls to about 0.5 mL/min at 1200 mOsm/kg; with none it can reach 15-20 mL/min at 50 mOsm/kg
- Independent of sodium
- This is free-water handling: it changes sodium concentration without changing total body sodium
- Sustained stimulation
- Prolonged ADH also increases AQP2 gene transcription, not only vesicle trafficking
When it goes wrong
Non-osmotic ADH release from pain, nausea, opioids or hypovolaemia, combined with hypotonic maintenance fluid
Hospital-acquired hyponatraemia — the reason maintenance fluids should be isotonic and the sodium checked daily
Practise this structure
1 question in the bank tagged Water Balance.