NDT Advance Access originally published online on April 3, 2008
Nephrology Dialysis Transplantation 2008 23(7):2147-2153; doi:10.1093/ndt/gfn049
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© The Author [2008]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Free water transport, small pore transport and the osmotic pressure gradient three-pore model of peritoneal transport
Department of Nephrology, University Hospital of Lund, Sweden
Correspondence and offprint requests to: Bengt Rippe, Department of Nephrology, Lund University, University Hospital of Lund, S-211 85 Lund, Sweden. Tel: +46-46-171247; Fax: +46-46-2114356; E-mail: bengt.rippe@med.lu.se
Keywords: aquaporins; capillary permeability; interstitium; lymphatic absorption; ultrafiltration
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Introduction |
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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
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Key physiologic features of the TPM |
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The capillary wall is the dominating peritoneal barrier, being heteroporous. Despite reflection coefficients (
) near zero, sieving coefficients (
) for small solutes are 0.5–0.6 and not near unity [(1–)]Transendothelial macromolecule transport occurs by convection through large pores and not by ‘transcytosis’
The reabsorption of isotonic fluid from the peritoneal cavity to plasma occurs via the small capillary pores due to the Starling mechanism, because of the high plasma to peritoneal colloid osmotic pressure gradient (
) during PDThe clearance of a macromolecular marker from the peritoneum to peritoneal tissues (KE) is a complex parameter determined by several different processes: convection into the tissue, ‘volume recirculation’ between tissue and cavity, lymphatic reabsorption, and capillary small pore fluid, but not macromolecule, reabsorption
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How does an endothelial glycocalyx affect the TPM? |
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Why is pore theory preferred to glycocalyx theory in the TPM?
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Conclusions |
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  O. Devuyst and E. Goffin Water and solute transport in peritoneal dialysis: models and clinical applications Nephrol. Dial. Transplant., July 1, 2008; 23(7): 2120 - 2123. [Full Text] [PDF]
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