The reasoning: separate salt from water, then multiply
- Thin descending limb: water‑permeable (AQP1), relatively low NaCl permeability → fluid concentrates.
- Thin ascending limb: water‑impermeable; passive NaCl exit (inner medulla).
- Thick ascending limb (TAL): water‑impermeable; apical NKCC2 (Na⁺–K⁺–2Cl⁻), apical ROMK K⁺ recycling (creates lumen‑positive voltage), basolateral ClC‑Kb/barttin and Na⁺/K⁺‑ATPase. Reabsorbs ~25% of filtered Na⁺; lumen‑positive potential drives paracellular Ca²⁺ and Mg²⁺ reabsorption (claudin‑16/19). The diluting segment — fluid leaves at ~100 mOsm/kg.