The concentration of urine depends upon $........$.

Mention the substances that exit from the tubules in order to maintain a concentration gradient in the medullary interstitium.

The increase in osmolarity from outer to inner medullary interstitium is maintained due to:
$(i)$ Close proximity between Henle's loop and vasa recta
$(ii)$ Counter current mechanism
$(iii)$ Selective secretion of $HCO_{3}^{-}$ and hydrogen ions in $PCT$
$(iv)$ Higher blood pressure in glomerular capillaries

The concentration of sodium and chloride ions is lowest in which of the following regions?

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.

The human kidneys produce urine that is approximately .......... times more concentrated than the initial filtrate.