The main function of Henle's loop is

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

If the kidneys fail to reabsorb water,what effect will be observed on the tissues?

$\text{NaCl}$ from the ascending limb of Henle's loop is exchanged with:

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.

Mammals have the ability to produce