Answer
The high osmolarity of interstitial fluid (ISF) in the renal medulla is due mainly to concentration of Na+, CL- and urea.
Work Step by Step
The osmolarity of urine increases about four-fold from the renal cortex to mid-renal medulla (300 mOsm/liter --1200 mOsm/liter).. This concentration is achieved by the effects of differences in permeability and water reabsorption along the nephron loop and collecting duct, and by counter current mechanisms (counter current multiplication).
