The flow of $P$ in the limbs of the Loop of Henle and the flow of $Q$ in the limbs of the vasa recta create a counter-current mechanism.
$\quad P \quad\quad Q$

Explain the mechanism of formation of concentrated urine in mammals.

Compare the statements $A$ and $B$.
$\text{Statement } A$: When the urine moves through the descending limb,it becomes hypertonic,and as it passes through the ascending limb of Henle's loop,it becomes hypotonic.
$\text{Statement } B$: The descending limb is permeable to sodium ions,while the ascending limb is impermeable to sodium ions.

Which structures take part in the counter-current mechanism?

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

Mammals have the ability to produce