The osmotic concentration in the interstitial fluid is primarily due to the presence of $.....$.

$A :$ 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 $H_2O$.

$A$: The final reabsorption of water from urine into blood occurs through the collecting duct of a mammalian nephron,resulting in the production of hyperosmotic urine.
$R$: The loop of Henle is responsible for the formation of a sodium gradient across the depth of the medullary interstitium of a mammalian kidney.

In the nephron, the transport of substances like sodium chloride and urea is facilitated by a special arrangement called the counter-current mechanism, which comprises of:

The counter-current multiplier system changes the isotonic glomerular filtrate into

At the bottom of the loop of Henle,the glomerular filtrate is . . . . . . .