To maintain the required osmotic concentration in the medullary interstitium of the kidneys of ureotelic animals,some amount of ..... is retained.

The counter-current mechanism operates in:

The main function of the loop of Henle is

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.

Indicate whether the following statement is true or false:
Henle's loop plays an important role in concentrating the urine.

Which ions enter the vasa recta?