A dialyzer is an artificial kidney designed to provide controllable transfer of solutes and water across a semi permeable membrane separating flowing blood and dialysate streams. The transfer processes are diffusion (dialysis) and convection (ultrafiltration*).
Blood cells, protein and other important things remain in your blood because they are too big to pass through the membrane. Smaller waste products in the blood, such as urea, creatinine, potassium and extra fluid pass through the membrane and are washed away.
*Ultra filtration: All excess fluid must be removed from the bloodstream as the patient’s blood flows through the dialyzer. The process of water removal from the bloodstream is called ultra filtration, and the amount of fluid removed is the ultra filtrate.