How dialysis works

Dialysis performs two functions that the kidneys are no longer able to do – removing excess waste and balancing fluid levels in the body.

Filtering waste products

Excess wastes and water pass from the blood into a special liquid called the dialysis fluid for removal from the body by a process called diffusion.

A thin layer of natural tissue (in Peritoneal Dialysis) or of synthetic plastic (in Haemodialysis), known as the dialysis membrane, keeps the blood apart from the dialysis solution (also called the dialysis fluid, or the dialysate).
Blood cells are too big to pass through the dialysis membrane, but wastes and water can diffuse through (that is, travel across) it into the dialysis solution.
Diffusion is complete. Body wastes have diffused through the membrane. There are equal amounts of waste in the blood and the dialysis solution.
Wastes are removed by removing the dialysis fluid.

Removing excess fluid

In both haemodialysis and peritoneal dialysis, the process called ultrafiltration removes excess fluid. This process occurs in slightly different ways in haemodialysis and in peritoneal dialysis.

In haemodialysis, the water is simply ‘sucked’ from the blood by the kidney machine. The amount of water to be removed during dialysis can be varied, depending on how the machine is set up.

In peritoneal dialysis a substance (usually glucose) is put into the blood, which ‘sucks’ the water from the blood using a process called osmosis. Peritoneal dialysis solution that contains a lot of glucose will draw off more fluid than a solution that is weaker.

May 1, 2006

Next: Haemodialysis