Human urine is usually . . . . . . compared to human blood.

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

Which of the following statements about the reabsorption process in Henle's loop are correct?
$(a)$ The descending limb of Henle's loop is permeable to water but almost impermeable to electrolytes.
$(b)$ Urine gets concentrated in Henle's loop.
$(c)$ Reabsorption of $Na^+$ and water takes place in Henle's loop.
$(d)$ Active or passive transport of electrolytes occurs in the ascending limb of Henle's loop.
Choose the correct answer from the options given below:

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:

Describe the counter-current mechanism.

$A :$ 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 $H_2O$.