Urine is concentrated byc) facilitated diffusion in the collecting ducts. The collecting duct concentrates it by water reabsorption through facilitated diffusion when ADH is present.The permeability of the collecting ducts for water leads to a concentration of the urine up to the fivefold osmolarity of the plasma.The permeability of the collecting ducts is regulated with ADH. Solution Urine is concentrated byc) facilitated diffusion in the collecting ducts. The collecting duct concentrates it by water reabsorption through facilitated diffusion when ADH is present.The permeability of the collecting ducts for water leads to a concentration of the urine up to the fivefold osmolarity of the plasma.The permeability of the collecting ducts is regulated with ADH..