Question

The high osmolarity of the renal medulla is maintained by

Answer 1

The osmolarity in renal cortex and medulla is mainly maintained by NaCl and urea. NaCl is transported from ascending limb of Henle's loop to descending limb of vasa recta(part of efferent arteriole which surrounds Henle's loop) . NaCl then goes back to interstitium( space between Henle's loop and vasa recta) through ascending part of vasa recta.

As descending limb of Henle's loop is impermeable to electrolytes ( and thus NaCl) it remains there, which increases the concentration in renal medulla. To match the concentration in interstitium ( to be in equilibrium) water molecules are drawn out from the descending limb of Henle's loop.

Small amounts of urea is also transported to interstitium ( drawn or pushed from collecting duct) and then to ascending limb of Henle's loop ( from where it returns to collecting duct).

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