Which feature enables the mammalian kidney to concentrate urine in the medullary region?

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.

Medullary gradient is mainly developed due 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:

The counter-current mechanism operates in:

Which part of the nephron plays a significant role in the maintenance of high osmolarity of the interstitial fluid?