Explanation

The counter-current mechanism involves two components:

1. Counter-current multiplier — Loop of Henle (creates the gradient by differential permeability and active NaCl pumping)
2. Counter-current exchanger — Vasa recta (maintains the gradient by recycling solutes and water without washing them away)

Flowchart: Counter-Current Mechanism

Key Points

• Without ADH: collecting duct remains impermeable → dilute urine produced regardless of medullary gradient.
• Without vasa recta counter-current exchange: medullary gradient is washed away.
• NaCl (from ascending limb) + Urea (from inner medullary collecting duct) = two main contributors to medullary gradient.
• Maximum human urine concentration: ~1200 mOsm/L.
• Kangaroo rat maximum: ~9000 mOsm/L (due to longer loops of Henle).