The prime driving force for countercurrent multiplier system is

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