The high sodium content in renal medulla is because

In the renal medulla, there are two mechanisms that work to initiate and maintain a hyperosmolar state. Counter current multiplier system and Counter current exchange system. Na+ is actively reabsorbed from the tubules to medullary interstitium, along with Cl- and K+, at the thick ascending limb of the loop of Henle, which is impermeable to water. So the medullary interstitium becomes hyperosmolar. Urea is also believed to help in increasing the solute content of the medullary interstitium by diffusing out of the collecting ducts and thus increasing the osmolarity of the interstitium. Water, in turn, is reabsorbed in the thin descending limb of the loop of Henle by osmosis. The cycle continues to work so that the medullary interstitial osmotic pressure is multiplied over and over. Counter current exchange system by the vasa recta maintains this osmotic pressure, as they do not wash out the solutes concentrated in the medullary interstitium.