Osmolarity gradient

Refers to the gradient created in the nephrons of the kidneys, between the tubules and the interstitial fluid surrounding them. It is created by the solutes (urea and salt) exiting and reentering the tubules at different sections of the nephron. The selective permeability of the tubules is a major factor in this process. Tissue osmolarity increases from cortex (outer layer of a nephron), to inner renal medulla (middle layer of a nephron).