Diffusion is a therapy that removes water and toxins from the body. Dialysis is typically applied in patients with acute or chronic kidney failure. The major forms of dialysis are
- Hemodialysis: Fluid and toxin removal from blood by diffusion and convection across a semipermeable membrane by sending the patient’s blood through an external filter
- Peritoneal dialysis: Fluid and toxin removal by diffusion and convection by instilling and removing fluid into the peritoneal cavity through an abdominal catheter
Diffusion is defined as the movement of solute from an area of high concentration to an area of lower concentration across a semipermeable membrane. Movement continues until equilibrium is achieved. The process was first described 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). In convective transport, the solvent (i.e., water) carries dissolved solutes across the membrane through all of the pores except the water channels (aquaporins).
The hemodialysis procedure uses an artificial, extracorporeal, semi-permeable membrane to separate a flowing stream of blood from a flowing stream of dialysis fluid or dialysate (1). The unwanted solutes that are present in the uremic blood in high concentrations move across the membrane down their concentration gradient into the dialysate 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 instilling and removing dialysis solution in the peritoneal cavity using a peritoneal catheter (2). In PD, fluid removal is achieved by osmosis and solute removal (clearance) by means of diffusion and convection. Several techniques and modalities of peritoneal dialysis are available (see 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.
1. Scneditz D. Technological aspects of hemodialysis and peritoneal dialysis. In: Clinical Dialysis, Nissesson AR, Fine RN Eds, 4th edn, 2005, McGraw-Hill, New York, pp 47-83.
2. Blake PG, Daugirdas JT. Physiology of peritoneal dialysis. In: Handbook of dialysis. Daugirdas JT, Blake PG, Ing TS, Eds, 4th edn, 2007, Philadelphia, pp 323-338.
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