As the descending limb of the loop of Henle is freely permeable to water, water diffuses out of this portion and into the medullary interstitium. This water is also quickly taken up by the capillaries.

Thus, these 4 factors would produce an osmolitity gradient from as low as 300mOsm/L in the cortex to as high as 1200mOsm/L at the tip of the loop of henle.