Dialysis is the diffusion of solutes through a semi-permeable membrane driven by a concentration gradient. The process was first named by Thomas Graham in 1861. This process has been ingeniously used in the treatment of uremic patients by utilizing an artificial membrane (hemodialysis) or a biological membrane (peritoneal dialysis). The hemodialysis procedure uses an artificial, extracorporeal, semi-permeable membrane to separate a flowing stream of blood from a flowing stream of dialysate. The unwanted solutes that are present in the uremic blood in high concentrations move across the membrane and are eliminated in the spent dialysate. Large molecules, such as most blood proteins and formed blood components are retained in circulation. Peritoneal dialysis is a therapy by which excess fluid and metabolic by-products are removed from the body by circulating dialysis solution through the peritoneal cavity using a peritoneal catheter(s).
Fluid removal in PD (see How to Achieve Adequate PD Ultrafiltration) is achieved by osmosis and solute removal (clearance) by means of diffusion. Several techniques and modalities of peritoneal dialysis have been developed (see PD Techniques and PD Modalities).
In hemodialysis, the artificial membrane is well characterized and both blood and dialysate flow can be controlled. In contrast, the peritoneal membrane functional characteristics vary among individuals and with time, and the peritoneal blood flow cannot be practically measured.