Final answer:
Urea is reabsorbed in the collecting duct of the kidney through facilitated diffusion, a process enhanced by the hormone ADH which promotes the incorporation of urea transporters and aquaporin channels.
Step-by-step explanation:
The subject of this question is biology, specifically the physiology of the renal system and the process by which urea is reabsorbed in the kidney. Urea is reabsorbed in the collecting duct primarily through a process known as facilitated diffusion. This is part of the kidney's function to maintain homeostatic plasma concentrations of various substances. According to physiological principles, when Antidiuretic Hormone (ADH) is present, it induces the insertion of urea transporters and aquaporin channel proteins, which allows for the reabsorption of urea into the bloodstream without the expenditure of energy, as would be the case in active transport.