text: what is the function of the vasa recta in maintaining medullary osmolarity in the kidney? i've tried a few websites but...still not getting it...from what i understand the point is to bring oxygen and nutrients into the medulla (which has higher metabolic needs than the cortex). the reason the vasa recta loops is so that sodium taken in/water taken out by the descending vasa recta can be reversed in the ascending vasa recta, thereby not changing the osmotic gradient in the medulla. is there more to it than this? just seems too easy lol. thanks! blood entering the interstitium in the vasa recta is 300 mosmol/l. as with capillaries elsewhere, these capillaries are permeable to salt and water.  as blood flows, in the vasa recta, from the cortex to the medulla, it equilibrates at each level with the salt and water which is drawn in from the interstitium by the net inward pressure (15mmhg).  as the blood returns to the cortex, the same process of equilibration occurs. as the blood leaves to enter the venules, its osmolarity is about 310 mosmol/l., just slightly hyper-osmotic with respect to plasma.  the vasa recta continuously take away a small amount of salt (osmolarity of 310 vs 300) and water from the interstitium but do not dissipate the gradient.