In a model of the medulla, the cortex is at 300 mOsmol/L and each level below it adds 150 mOsmol/L…

The osmolarity of the interstitial fluid rises from $300$ mOsmol/L in the cortex to about $1200$ mOsmol/L in the inner medulla. The descending limb is permeable to water. The ascending limb is impermeable to water and lets electrolytes out. The vasa recta runs beside the loop in a counter current. Only water leaves the collecting duct, so the amount of solute stays the same: $\text{volume} \times \text{osmolarity}$ is constant.