Figure 47

Fluid removal by ultrafiltration. During peritoneal dialysis, hyperosmolar glucose solution generates ultrafiltration by the process of osmosis. Water movement across the peritoneal membrane is proportional to the transmembrane pressure, membrane area, and membrane hydraulic permeability. The transmembrane pressure is the sum of hydrostatic and osmotic pressure differences between the blood in the peritoneal capillary and dialysis solution in the peritoneal cavity. Net transcapillary ultrafiltration defines net fluid movement from the peritoneal microcirculation into the peritoneal cavity primarily in response to osmotic pressure. Net ultrafiltration would equal the resulting increment in intraperitoneal fluid volume if it were not for peritoneal reabsorption, mostly through the peritoneal lymphatics. Peritoneal reabsorption is continuous and reduces the intraperitoneal volume throughout the dwell. A, The net transcapillary ultrafiltration rate decreases exponentially during the dwell time, owing to dissipation of the glucose osmotic gradient secondary to peritoneal glucose absorption and dilution of the solution glucose by the ultrafiltration. Later in the exchange net, ultrafiltration ceases when the transcapillary ultrafiltration is reduced to a rate equal to the peritoneal reabsorption. B, When the transcapillary ultrafiltration rate decreases below that of the peritoneal reabsorption rate, the net ultrafiltration rate becomes negative. Consequently, the intraperitoneal volume begins to diminish. Thus, peak ultrafiltration and intraperitoneal volumes are observed before osmotic equilibrium during an exchange.

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