Relevance of three pore model to peritoneal membrane transport
Fluid and solute are transported between the blood and the peritoneal cavity across the peritoneal membrane.
The capillary endothelial membrane provides the primary hindrance to this exchange. Mathematical modeling to describe, understand, and simulate this transport was captured by Bengt Rippe in his 1991 description of the “Three-Pore Model.” The accuracy of the model has been validated in clinical studies and is used extensively to predict solute and fluid removal using different PD prescriptions for given peritoneal transport characteristics. As implied, transport across the peritoneal capillary is characterized to occur across three distinct endothelial “pores”:
1. Intracellular aquaporins or water channels. Aquaporins are affected by osmotic pressure and are exclusively permeable to water.
2. Inter-endothelial cell or “small” pores. Small pores respond to both crystalloid and colloid osmotic forces and are permeable to both water and solutes smaller than albumin.
3. Large inter-endothelial cellular pores. Large pores account for less than 0.01% of all capillary pores. While these are capable of passing larger molecules and proteins, they are effectively unresponsive to osmotic forces given their large size (precluding a transcellular gradient). Transport across these is unidirectional from plasma to peritoneal cavity and occurs by hydrostatic pressure. Large pores are responsible for leakage of protein into the peritoneal cavity.
The Three-Pore Model is a fairly accurate mathematical tool used to predict solute and water transport for specific peritoneal membrane transport characteristics in response to different PD solutions with varying osmotic contents.