As glomerular filtrate moves through the ascending loop of Henle, the filtrate becomes more dilute. The reason is the ascending limb of loop of Henle is

The thick ascending limb of the loop is lined by the cells which have symporters that actively reabsorb Na⁺, K⁺, and Cl⁻ from the filtrate. At the same time the thick ascending limb is almost impermeable to water and so water cannot follow by osmosis in this region of nephron. Because only the solutes are leaving the filtrate (and not the water), the osmolarity drops to about 150 mOsmL^-1. Thus, the fluid entering the DCT is more dilute than plasma.