Formation of hypertonic urine is facilitated by

Formation of hypertonic urine is mediated through counter current mechanism. 

The parallel flow of filtrate (countercurrent) through the descending and ascending limbs of the long nephron loop establishes the osmotic gradient in the renal medulla. The long Henle's loop acts as countercurrent multiplier and establishes the osmotic gradient. This osmotic gradient helps in the concentration of urine.