State the importance of the following:
$(1)$ Sebum
$(2)$ Counter current mechanism

The main function of Henle's loop is

The Loop of Henle is primarily responsible for the reabsorption of which of the following?

If the $Loop$ $of$ $Henle$ is absent from the mammalian nephron,which of the following is likely to occur?

In vertebrates,the ability to produce concentrated urine is generally associated with the ............ .

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