What does the following figure represent?

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

Give a brief account of the counter current mechanism.

The shape of vasa recta is

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