Chapter 26, Lecture 6

Nephron Loop (Loop of Henle)

  • Proximal tubule already reabsorbs bulk of filtrate; loop handles remaining solute/water adjustments.
  • Overall reabsorption (approx.):
    • 35%35\% Cl−^-
    • 25%25\% Na+^+, K+^+, Ca2+^{2+}
    • 50%50\% water (descending limb only)
    • 10%10\% HCO3−_3^-
    • Variable Mg2+^{2+}
  • Key concept: uncoupled solute & water movement → independent control of volume vs. osmolarity → basis for urine concentration.

Segment Properties

  • Descending limb: permeable to water, impermeable to solutes.
  • Thick ascending limb: impermeable to water; contains Na+/K+/2Cl−\text{Na}^+\text{/K}^+/2\text{Cl}^- symporter.

Thick Ascending Limb Transport

  • Symporter moves: 1 Na+^+ (down gradient), 1 K+^+ + 2 Cl−^- (against).
  • Na+^+ exits cell via Na+/K+\text{Na}^+/\text{K}^+ ATPase → vasa recta.
  • Cl−^- exits via channels → blood.
  • K+^+ leaks back into tubular fluid → net reabsorption = NaCl only.
  • Resulting negative interstitium drives paracellular reabsorption of cations (Ca2+^{2+}, Mg2+^{2+}, Na+^+, K+^+).

Osmolarity Changes Along Loop

  • Down descending limb: water leaves → tubular fluid becomes concentrated ↑ osmolarity.
  • Up ascending limb: NaCl pumped out, water retained → tubular fluid diluted ↓ osmolarity.

Early Distal Convoluted Tubule (DCT)

  • Reabsorbs ~5%5\% remaining NaCl via Na/Cl symporter.
  • Major site for Ca2+^{2+} reabsorption regulated by PTH\text{PTH}.

Late DCT & Collecting Duct (Fine-Tuning)

  • ~5−10%5{-}10\% filtrate components remain; segment adjusts according to body needs.
  • No symporters/antiporters; ions move via leakage channels + diffusion.

Principal Cells

  • Reabsorb Na+^+, secrete K+^+.
  • Regulated by aldosterone & ADH\text{ADH}.
  • Main source of urinary K+^+ elimination.

Intercalated Cells

  • Type A: reabsorb HCO3−_3^-, secrete H+^+.
  • Type B: secrete HCO3−_3^-, reabsorb H+^+.
  • Both reabsorb K+^+; key for acid–base balance.

Summary of Water Handling

  • Obligatory water reabsorption: proximal tubule (solute-coupled).
  • Facultative/regulated water reabsorption: descending limb (passive) + collecting duct (via ADH\text{ADH}).
  • Thick ascending limb & early DCT: water impermeable, enabling urine concentration mechanism.