Kidney Production of Hypertonic Urine: Mechanisms of Concentration

The kidney produces hypertonic urine (up to 1200–1400 mOsm/kg) from isotonic plasma (~300 mOsm/kg) via a countercurrent multiplier system.

Core Mechanisms

1. Countercurrent Multiplication (Loop of Henle)

• Descending Limb: Water-permeable (AQP1), solute-impermeable → water exits passively into hypertonic medulla.
• Thin Ascending Limb: NaCl passively reabsorbed (paracellular).
• Thick Ascending Limb (TAL): Active Na⁺-K⁺-2Cl⁻ cotransport (NKCC2) → dilutes tubular fluid, builds medullary gradient.

2. Distal Convoluted Tubule (DCT) & Connecting Tubule

• Na⁺-Cl⁻ cotransport (NCC) – thiazide-sensitive.
• Regulated by aldosterone (ENaC channels) and PTH (Ca²⁺ reabsorption).

3. Collecting Duct & ADH (Vasopressin)

• ADH inserts AQP2 water channels → water reabsorption down medullary osmotic gradient.
• Urea recycling (UT-A1/3 transporters) maintains inner medullary tonicity.

4. Countercurrent Exchange (Vasa Recta)

Preserves medullary gradient by minimizing solute washout.

Regulation

• Plasma osmolality → hypothalamic osmoreceptors → posterior pituitary ADH release.
• Volume/pressure → baroreceptors → ADH (non-osmotic stimulus).
• RAAS → aldosterone → Na⁺/water retention.

This intricate system allows water conservation independent of solute excretion.