Medullary gradient is mainly developed due to

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

The descending limb of the loop of Henle is permeable to ........ .

The shape of vasa recta is

Name the following:
$A$ loop of capillary running parallel to the Henle's loop.

$A :$ In the descending limb of the loop of Henle,the urine is hypertonic,while in the ascending limb of the loop of Henle,the urine is hypotonic.
$R :$ The descending limb is impermeable to $Na^+$,while the ascending limb is impermeable to $H_2O$.