Solute and Water Transport in Peritoneal Dialysis: A Case-Based Primer

Peritoneal dialysis (PD) is an effective therapy for patients with end-stage kidney disease. Dialysis solutions containing physiologic concentrations of electrolytes and base, as well as glucose often at supraphysiologic concentrations, are infused into the peritoneal cavity for solute and water exchange, and the patient's own peritoneal membrane is used for dialysis. The peritoneal membrane is dominated by small pores, which allow transport of water and small-molecular-size solutes, including electrolytes, by way of both diffusion and convection. Through small pores, diffusion allows the movement of solutes from the high-concentration compartment to a lower-concentration region. Also, through small pores, water and solutes move together by convection in response to an osmotic force. The glucose in the dialysis solution generates osmotic force to drive convection. In addition to small pores, the peritoneal membrane contains a specialized water channel, aquaporin 1, which is also present in capillaries of the peritoneal membrane. These specialized water channels, which are upregulated by glucose, allow water transport without solute (free water) in response to the osmotic force induced by glucose in the PD solution. During a PD exchange, net loss or gain of electrolytes and base is determined by both their gradient between capillary blood and dialysis solution and the net ultrafiltration volume. Developing a PD prescription, including the amount of glucose used, and changing the prescription in response to dietary changes and/or loss of residual kidney function requires a sound understanding of the peritoneal physiology. The case studies presented here help solidify the basic elements of PD prescription and how the PD prescription should be altered in response to changing clinical situations.