Medullary gradient is mainly developed due to

Why is a small amount of urea retained in the kidney matrix?

Given below are two statements with respect to counter current mechanism.
$Statement-I$: Tissue fluid around the descending limb of Henle's loop becomes concentrated, during the counter current mechanism.
$Statement-II$: Water moves out from the descending limb of Henle's loop into the tissue fluid by osmosis.
In the light of the above statements, select the correct option from the codes given below:

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

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

Which structures take part in the counter-current mechanism?