The counter-current multiplier system changes the isotonic glomerular filtrate into

We can produce a concentrated urine. This is facilitated by a special mechanism. Identify the mechanism.

Henle's loop of nephron plays a significant role in maintaining a high osmolarity in

The increase in osmolarity from outer to inner medullary interstitium is maintained due to:
$(i)$ Close proximity between Henle's loop and vasa recta
$(ii)$ Counter current mechanism
$(iii)$ Selective secretion of $HCO_{3}^{-}$ and hydrogen ions in $PCT$
$(iv)$ Higher blood pressure in glomerular capillaries

The vasa recta are tubular capillaries around the ........

The concentration of urine depends upon which part of the nephron?