A Three-Pore Model of Peritoneal Transport

Abstract

The three-pore model of peritoneal transport treats the capillary membrane as a primary barrier determining the amount of solute that transports to the interstitium and the peritoneal cavity. According to the three-pore model, the principal peritoneal exchange route for water and water-soluble substances is a protein-restrictive pore pathway of radius 40–55 A, accounting for approximately 99% of the total exchange (pore) area and approximately 90% of the total peritoneal ultrafiltration (UF) coefficient (LpS). For their passage through the peritoneal membrane proteins are confined to so-called “large pores” of radius approximately 250 Å, which are extremely few in number (0.01% of the total pore population) and more or less nonrestrictive with respect to protein transport. The third pathway of the three-pore model accounts for only about 2% of the total LpS and is permeable to water but impermeable to solutes, a so-called “water-only” (transcellular?) pathway. In contrast to the classical Pyle-Popovich (P&P) model, the three-pore model can predict with reasonable accuracy not only the transport of water and “small solutes” (molecular radius 2.3–15 Å) and “intermediatesize” solutes (radius 15–36 Å), but also the transport of albumin (radius 36 Å) and larger molecules across the peritoneal membrane. The model operates with reflection coefficientsa (a's) for small solutes <0.1. These are approximately one order of magnitude lower than the & sigma's In the P&P model. Furthermore, the peritoneal LPS is one order of magnitude higher than In the P&P model. As a consequence, the major portion of the “fluid loss” from the peritoneal cavity In continuous ambulatory peritoneal dialysis (CAPD) can be explained by the operation of the so-called Starling forces (the transcapillary hydrostatic pressure gradient opposed by the plasma colloid osmotic pressure as multiplled by the LpS), and to a much lesser extent by lymphatic absorption (L). Furthermore, In contrast to the P&P model, the three-pore model can with reasonable accuracy predict the UF profiles produced when glucose Is substituted by high molecular weight solutes as osmotic agents In CAPO.