In haemodialysis the main processes occur between the patient's blood and the dialysate via the dialyser membrane. The patient's blood flows on one side of the dialyser membrane; the dialysate flows in the opposite direction on the other side. Diffusion, filtration, osmosis and convection processes take place between blood and dialysate at this point via the semi-permeable membrane. This is made possible by differences in concentration, hydrostatic pressure, osmotic pressure and flow rate.

Dialysate consists of purified water and various substances dissolved in it. With the exception of glucose, the substances dissolved in the dialysate are all electrolytes. Their concentration (apart from potassium and the buffer substance) closely resembles the concentration of the electrolytes occurring naturally in the blood. Dialysate regulates the electrolyte and acid-base balance of the dialysis patient and removes waste products.

Dialysate is prepared in the dialysis machine automatically by continuous mixing of haemodialysis concentrates and purified water in a fixed ratio.