A Two Pore Size Distribution Model for Transcapillary Exchange of Substances

Abstract

Net transcapillary fluid exchange results from an imbalance between the hydrostatic and osmotic forces acting across the microvascular exchange barrier. Net solute exchange results primarily as a passive phenomena in response to a concentration gradient across the microvasculature. Morphological (12)(13) as well as physiological (3)(4) measurements support the notion that the endothelial barrier of the microvasculature, through which such exchange must occur, is structurally heterogeneous. Several pathways for fluid and solute exchange have been proposed which include; direct passage through the endothelial plasma membranes (and intervening cytosol), a “pore” which is formed by the fusion of vesicles which creates a large channel through the endothelial cell; and, the pathway between the endothelial cells at the endothelial cell junctions. The latter have generally been considered the morphological counterpart of the “small-pore” and “large-pore” systems, while the first two pathways are considered to contribute minimally (10–15%) to overall blood to tissue transport of fluid and solutes. The initial attempt, made by Pappenheimer, Renkin and Borrero (11), was to characterize the movement of fluid and solute with a single, small-pore pathway model.