As the filtrate passes back up the<unk> , the concentration of the surrounding medulla decreases. The<unk><unk>  limb is not permeable to water, but it is permeable to ions.
Sodium ions (Na+) are passively transported down this concentration gradient, from the<unk><unk>  limb into the medulla.
Thin ascending limb of loop of Henle
Thick ascending limb of loop of Henle
Loop of Henle
Renal tubule
Distal convoluted tubule
Renal medulla
Renal corpuscle
Peritubular capillaries