Answer:
The kidney's ability to concentrate urine depends on the maintenance of an osmolarity gradient between the interstitial fluid of two structures: the cortex and the medulla.
Explanation:
In order to concentrate urine, the kidney needs to move some substances through the medulla and the cortex. Due to this movement, osmolarity gradient is developed. The osmolarity gradient is usually maintained by the passive transport of urea and the active transport of NaCl. This will cause the water to move from an area of higher concentration gradient to an area of lower concentration gradient.
