In our kidneys, the concentration of medullary interstitium 

In our kidneys, the concentration of medullary interstitium increases from 300 mOsmL^-1 in the outer medulla to 1200 mOsmL^-1 in the inner medulla. This concentration gradient in the medullary interstitium of the kidneys is crucial for the process of water reabsorption. It creates an osmotic gradient that allows the reabsorption of water from the renal tubules, especially in the collecting ducts. As the filtrate passes through the renal tubules, the increasing concentration of the medullary interstitium draws water out of the tubules, helping to concentrate urine and regulate the body's water balance.