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Counter-current mechanism of kidney
Fluid entering the loop of Henle from one direction creates an osmotic pressure gradient.
The interplay with the blood flowing in the opposite direction in the vasa recta constitutes the counter-current mechanism.
The isotonic fluid entering the loop of Henle contains urea, water, ##"K"^+, "Na"^+##, and ##"Cl"^"-"## ions.
Blood flows through the surrounding capillaries (the vasa recta) in the opposite (counter-current) direction.
The descending limb of the loop is not permeable to ions, but it absorbs urea and is permeable to water.
Water moves out of the limb by osmosis and passes into the vasa recta. Solutes remain behind.
The concentration increases by a factor of four. The solution becomes hyperosmotic (high osmotic pressure).
The ascending limb is impermeable to water, but it is permeable to ions.
##"Na"^+, "K"^+##, and ##"Cl"^"-"## ions move by osmotic pressure across the membrane and back into the blood.
The concentration of the liquid decreases by a factor of twelve. The solution becomes hypotonic (low osmotic pressure).
Here is an excellent animation of the process