Figure 1.

Schematic representation of four-region axially distributed single capillary-tissue unit composed of plasma at flow F_p and surrounding capillary endothelial wall, interstitial fluid space, and parenchymal cells of the organ. The Vs are volumes of distribution. Barrier conductances are given by the permeability surface area product (PS). The capillary wall is permeated by passive transport through interendothelial clefts as well as transport across the endothelial plasmalemma. Axial dispersion (D) approximately accounts for intravascular velocity profiles and molecular diffusion. Intra-regional reactions or metabolic consumption are given by the “gulosities” or clearances (G). p, plasma; g, interendothelial cleft or gap PS; ecl, luminal surface of endothelial cell; ec, endothelial cell; isf, interstitium; eca, antiluminal surface of endothelial cell; pc, parenchymal cell.