Net ultrafiltration in peritoneal dialysis: role of direct fluid absorption into peritoneal tissue.

'Net ultrafiltration' in peritoneal dialysis refers to the difference between the osmotically induced ultrafiltration into the peritoneal cavity and the fluid loss from the cavity during dialysis. Recent research has demonstrated that, during a 3- to 4-hour experimental dialysis, 5-25% of the total fluid loss is via lymphatics and the remaining fluid is absorbed directly into the tissue surrounding the peritoneal cavity. The driving force for this convection into tissue is the hydrostatic pressure gradient between the peritoneal cavity and the tissue, which ranges from 2 to 8 mm Hg during the typical 2-liter dialysis in humans. Because the convection from the cavity occurs during periods of a positive net ultrafiltration, the peritoneum and its underlying tissue cannot be represented as a single membrane but function as a composite of 'tight' and 'loose' membranes. More data on the mechanical properties of the peritoneal tissue space and its response to hydrostatic pressure in the cavity are required before we fully understand fluid transport at the tissue level.