Facultative Water Reabsorption: How ADH Fine-Tunes Kidney Concentration

Facultative water reabsorption is the regulated, hormone-dependent process by which the kidneys adjust urine concentration to match body hydration needs. It occurs in the collecting ducts and is controlled by antidiuretic hormone (ADH, vasopressin).

The Mechanism

1. Baseline: Collecting duct cells are impermeable to water (tight junctions, no aquaporins).
2. Trigger: Plasma osmolality ↑ (dehydration) → hypothalamic osmoreceptors → posterior pituitary releases ADH.
3. Action: ADH binds V2 receptors on collecting duct principal cells → cAMP → PKA → translocation of aquaporin-2 (AQP2) water channels to apical membrane.
4. Result: Water moves passively down osmotic gradient (created by countercurrent multiplication in loop of Henle) from tubular lumen → interstitium → peritubular capillaries → blood.
5. Reversibility: When hydration restored, ADH falls → AQP2 internalized → collecting duct becomes impermeable again → dilute urine excreted.

Key Points

• Obligatory vs. facultative: Proximal tubule and descending loop reabsorb ~85% of filtered water obligatorily (isosmotic). Facultative reabsorption (5–15%) determines final urine volume/concentration.
• Dynamic range: Urine osmolality 50–1200 mOsm/kg; volume 0.5–20 L/day.
• Regulation: Primarily plasma osmolality (threshold ~285 mOsm/kg). Secondary: volume/pressure (baroreceptors) — hypotension overrides osmolality to preserve volume.

Clinical Relevance

• Diabetes insipidus: Central (no ADH) or nephrogenic (ADH resistance) → massive dilute urine, polydipsia.
• SIADH: Inappropriate ADH → water retention, hyponatremia, concentrated urine.
• Lithium: Causes nephrogenic DI by blocking AQP2 expression.

This elegant feedback loop allows precise water balance without sodium loss.