Question

what are the two substance responsible for causing day gradient for increasing hyperosmolarity of medullary interstitium​

Answer 1

The medullary interstitium is the tissue surrounding the loop of Henle in the renal medulla.

It functions in renal water reabsorption by building up a high hypertonicity, which draws water out of the thin descending limb of the loop of Henle and the collecting duct system. This hypertonicity, in turn, is created by an efflux of urea from the inner medullary collecting duct.[1]