An average dialyzer is an artificial filter with about 10,000 microscopic dialysis fibers, which are also known as dialysis membranes. The fibers are about 0.3 mm or 300 µm wide and are hollow with semi-permeable walls. Blood runs through the dialysis fibers and the dialysate is pumped in the opposite direction. When blood is guided through the inner lumen of the capillaries, toxins up to a defined molecular size may leave the blood stream through pores in the capillary wall to the outer space. Here they are removed by a rinsing solution. Vital substances and blood cells remain in the blood stream, they are held back by the tiny pores.

The transfer of metabolic toxins through the membrane into the dialysate is based on physical transport laws. When two liquids (in this case blood and dialysate) with differing concentrations of substances are separated by a semi-permeable membrane, molecules attempt to offset the concentration difference. The molecules move from one liquid to the other in a process known as diffusion. Proteins and blood cells would naturally also work to balance out the differences but, because of their comparatively big size and the tiny pores, they cannot pass through the membrane and are retained.



To remove excess water from the body, especially in peritoneal dialysis, fluids contain sugar molecules. The concentration of the sugar in the fluid is higher than in the blood. Since the sugar molecules cannot pass through the membrane, the only way of offsetting the difference in concentration is for water to pass through the membrane and thus to compensate for the concentration difference. This process is also known as osmosis. In osmosis, as opposed to diffusion, molecules move in only one direction. By continuously introducing fresh dialysis fluid containing sugar molecules, excess water that the kidneys could not filter out and that would have collected in the body, can be removed. Peritoneal dialysis is also based on the principles of osmosis and diffusion, with the patient’s abdominal lining acting as the body’s own dialysis membrane.