Which structures take part in the counter-current mechanism?

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 adaptation is shown by desert organisms?

Explain the mechanism of concentration of the filtrate.

Assertion : In the descending limb of the loop of Henle,the urine is hypertonic,whereas in the ascending limb of the loop of Henle,the urine is hypotonic.
Reason : The descending limb is impermeable to sodium,while the ascending limb is impermeable to water.

The concentration of sodium and chloride ions is lowest in which of the following regions?