The prime driving force for countercurrent multiplier system is: (A) Medullary hyperosmolarity, (B) Reabsorption of Na+ in thick ascending limb, (C) Action of ADH aquaporin channels, (D) Urea recycling
Answer is B. In juxtamedullary nephrons with longer loops and thin ascending limbs, the osmotic gradient is spread over a greater distance and the osmolality at the tip of the loop is greater. This is because the thin ascending limb is relatively impermeable to water but permeable to Na+ and Cl-. Therefore, Na+ and Cl- move down their concentration gradients into the interstitium, and there is additional passive countercurrent multiplication.Ref: Ganong&;s Review of Medical Physiology, 23rd edition, page no: 655