Mammals have the ability to produce

Why is a small amount of urea retained in the kidney matrix?

Vasa recta refers to . . . . . . .

$A :$ 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 $H_2O$.

The longest loop of Henle is found in

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