The contribution of free water transport and small pore transport to the total fluid removal in peritoneal dialysis

Kidney Int. 2005 Oct;68(4):1849-56. doi: 10.1111/j.1523-1755.2005.00604.x.


Background: Water transport in peritoneal dialysis (PD) patients is across the small pores and water channels, the latter allowing free water transport. The objective of the study was to investigate the contribution of each transport route on transcapillary ultrafiltration (TCUF).

Methods: Standard peritoneal permeability analyses of 80 stable PD patients were analyzed. Twenty-nine patients were followed longitudinally. Fluid transport through small pores (SPT) was assessed by the amount of transported sodium. Free water transport (FWT) was calculated by subtracting SPT from TCUF. The contribution of FWT and SPT to the TCUF and water transport rates at any time point was computed.

Results: The ultrafiltered volume due to SPT increased gradually during the dwell, while FWT reached its maximum around 3 hours. The rate of FWT decreased continuously during the dwell. SPT decreased during the initial 2 hours and remained stable thereafter. At 60 minutes, the SPT (P < 0.05) and its contribution (P < 0.05) were positively related to the MTAC(creat). The contribution of FWT after 1 hour, but not the absolute amount, showed an inverse relationship. Peritoneal solute transport parameters (P < 0.01) and the contribution of SPT (P= 0.08), but none of the other fluid parameters showed a U-shape with the lowest values in the second year of PD (P < 0.01).

Conclusion: The dwell courses of water transport suggest that the activity of water channels is dependent and limited by the crystalloid osmotic pressure gradient, while other determinants are important in SPT. The time-course of SPT paralleled that of peritoneal solute transport parameters.

MeSH terms

  • Adult
  • Aged
  • Aquaporins / metabolism
  • Body Fluids / metabolism
  • Dialysis Solutions / pharmacokinetics
  • Female
  • Humans
  • Kidney Failure, Chronic / metabolism*
  • Kidney Failure, Chronic / therapy*
  • Longitudinal Studies
  • Male
  • Middle Aged
  • Osmotic Pressure
  • Peritoneal Dialysis*
  • Peritoneum / metabolism*
  • Ultrafiltration
  • Water / metabolism*
  • Water-Electrolyte Balance / physiology


  • Aquaporins
  • Dialysis Solutions
  • Water