Free water transport, small pore transport and the osmotic pressure gradient three-pore model of peritoneal transport.

In this issue of NDT, Flessner in a commentary [1] argues that the three-pore model (TPM) of peritoneal transport, although mathematically a powerful predictor of solute transport and ultrafiltration (UF) in peritoneal dialysis (PD), may be too simple as a tool for understanding the physiology of transperitoneal exchange. Flessner then disregards the fact that the TPM can be modified in a very simple fashion by taking both the capillary and the interstitial barriers into account in the modelling. This has in fact already been done by adding a second heteroporous barrier [2] or an interstitial gel–matrix barrier in series with the capillary membrane in the TPM; the latter model denoted the ‘three-pore membrane/fibre matrix model’ [3]. Flessner also brings up now the 30-year-old controversy whether the endothelial ‘fuzzy’ surface layer, the glycocalyx, has size-selective sieving properties or not. In response to Flessner’s criticism of the TPM, I would like to cite Leonardo DaVinci: ‘Simplification is the ultimate form of sophistication’. The TPM is based on decades of basic capillary physiologic research, and the many simplifications made in the model have been done with due consideration of all the complexities of the peritoneal barrier. The usefulness of the model is illustrated by the fact that it can predict solute and fluid transport not only for glucose, but also for alternative osmotic agents, such as icodextrin and amino acid solutions [4,5], and also for solutions with altered electrolyte composition [6,7]. This is in contrast to the distributed model, which is in an early developmental phase. For example, it has great problems in predicting UF (glucose osmosis) from the peritoneal tissue to the peritoneal cavity. In contrast, the three-pore