Solute transport across the peritoneal membrane

The current understanding of the transport pathways that govern solute removal during peritoneal dialysis is reviewed. Diffusive transport rates across the peritoneal membrane for small solutes are slow. Even though the rate of diffusive solute transport decreases with increasing molecular size, large molecules (e.g., albumin) are nevertheless removed from the patient during routine peritoneal dialysis. Recent work has confirmed a previous suggestion that diffusive solute transport is limited by the small area of the peritoneal membrane that participates in the transport process. This small functional area is due to either poor contact of the peritoneal membrane with dialysis solution bathing the peritoneal cavity or to the limited surface area of capillaries that perfuse peritoneal tissues. Convective solute transport during peritoneal dialysis is proportional to the transperitoneal ultrafiltration rate but is less than that expected, because of low solute sieving by the peritoneal membrane and fluid absorption from the peritoneal cavity. Low solute sieving across the peritoneal membrane was first identified in 1966, a phenomenon that is now attributed to the presence of water-only transport pathways mediated by aquaporin-1. Fluid absorption from the peritoneal cavity occurs at the same time as transperitoneal ultrafiltration, but the pathways by which these two processes occur simultaneously remain speculative. This review proposes a novel hypothesis, whereby fluid absorption occurs in areas of the peritoneal membrane that are governed by different physical forces than those governing transperitoneal ultrafiltration. Further understanding of the pathways for fluid and solute transport during peritoneal dialysis will permit improvements in the adequacy of the dialysis dose and the more efficacious use of peritoneal dialysis to treat patients with end-stage renal disease.