Renal medullary hyperosmolarity is due to:

A i.e. Increased Na+The Counter current mechanism at the loop of henle is responsible for development of hyper osmolarity of medullary interstitial fluid and medullary gradient. Hyperosmolarity is due to high Na+ conc. of the filtrate in renal medullaQMaintenance of osmolarityMedullary gradientThe osmolarity of coicalinterstitium is same as that of plasma (300 mosm/L). However osmolality of medullary interstitium is very high (1200 mosm/L)Gradual increase in the osmolarity (from 300 to 1200 mosm/L) of medullary interstitium is called medullary gradientCounter - current mechanism isresponsible for maintenance of medullary gradient.