Artificial kidneys work by pumping blood through a special filter called a dialyzer. The blood is pumped into a compartment that is separated from the distillate compartment by a semi-permeable membrane. The distillate compartment is kept at a lower pressure than the blood compartment, allowing waste products in the blood to pass through the membrane. http://www.ask.com/wiki/Hemodialysis
An artificial kidney is in the form of a dialysis machine. This acts through the processes of osmosis and diffusion. Through these movements of solutes and solvents, electrolytes and waste products move in and out respectively from the blood.