Simplified schematic of how dialisys works:
Arterial blood from the patient enters the dialyser in many small tubes.
In the dialyser the blood is in many fine tubes and surrounded by a dialysate liquid that will ‘draw’ the impurities/waste chemicals from the blood via a semipermeable membrane and a pressure gradient between the blood and dialysate.
The ‘cleaned’ blood is returned to the venous system.
The dialysate is refreshed throughout.