The human kidneys produce urine that is approximately .......... times more concentrated than the initial filtrate.

In the given diagram,what are the regions $P, Q,$ and $R$?
$P \quad\quad Q \quad \quad R$

What does the following figure represent?

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

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

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