Final answer:
In dialysis, the upper component of the dialyzer, containing the dialysate, has a higher solute concentration than the blood. This allows for the diffusion of urea from the blood to the dialysate, facilitated by the concentration gradient.
Step-by-step explanation:
The concentration of solutes likely differs between the upper component of the dialyzer and the lower compartment, containing the fresh dialysate, in order to successfully remove wastes from the blood. In the upper component, the dialysate has a higher solute concentration than the blood, which allows urea to diffuse to the lower dialysate down its concentration gradient. This process happens through facilitated diffusion, where the higher concentration of solutes in the dialysate creates a concentration gradient that drives the movement of solutes like urea.