Urine Concentration & Volume Regulation

Plasma Osmolarity & Kidney Role

  • Blood plasma maintained near 300\ \text{mOsm/L}.
  • Only kidneys can correct plasma osmolarity by altering urine volume & concentration.

Hydration States & Urine Output

  • Dehydration (low intake / high loss):
    • Produce small volume of concentrated urine.
    • Reabsorb large fraction of tubular water.
  • Over-hydration (high intake):
    • Produce large volume of dilute urine.
    • Excrete excess water to restore 300\ \text{mOsm/L}.

Sites & Hormonal Requirement

  • Urine concentration occurs exclusively in juxtamedullary nephrons (long loops, vasa recta).
  • Cortical nephrons lack vasa recta → no concentrating ability.
  • Antidiuretic hormone (ADH):
    • Must be present to produce concentrated urine.
    • Absence → dilute urine regardless of hydration state.

Medullary Osmotic Gradient

  • Interstitial osmolarity rises from cortex (≈300) to inner medulla (≈1200\ \text{mOsm/L}).
  • Gradient composed mainly of \text{NaCl} and urea.

Countercurrent Multiplication (Loop of Henle)

  • Establishes the gradient.
  • Three key properties:
    1. Opposite flow directions in descending vs. ascending limbs.
    2. Descending limb: permeable to water, impermeable to \text{NaCl}.
    3. Ascending limb: pumps \text{NaCl} out (via \text{Na}^+ /\text{K}^+ /2\text{Cl}^- symporter), impermeable to water.
  • Result:
    • Ascending limb fluid becomes progressively dilute.
    • Descending limb fluid becomes progressively concentrated as water exits.
    • Repetition of “pump salt → water follows” multiplies difference until inner medulla reaches ≈1200\ \text{mOsm/L}.

Countercurrent Exchange (Vasa Recta)

  • Maintains the gradient.
  • Features:
    • Hairpin loop runs parallel to loop of Henle.
    • Permeable to both water & solutes; blood flow is slow.
    • Descending vasa recta: loses water, gains solute.
    • Ascending vasa recta: gains water, loses solute.
    • Net effect: solutes trapped in medulla; gradient not “washed out”.

Essential Conditions to Preserve Gradient

  • Loop architecture (nephron & vasa recta).
  • Differential permeability of limbs.
  • Slow medullary blood flow.
  • Presence of ADH when concentrating urine.

Quick Recap

  • Countercurrent multiplication = creates gradient.
  • Countercurrent exchange = preserves gradient.
  • Gradient + ADH → water reabsorption in collecting duct → concentrated urine when needed.
  • Disruption of any component impairs kidney’s ability to regulate plasma osmolarity.