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1 Departments of Biomedical and Mechanical Engineering, The City College of The City University of New York, New York, New York, United States
2 Department of Physiology and Membrane Biology, University of California at Davis, Davis, California, United States
* To whom correspondence should be addressed. E-mail: weinbaum{at}ccny.cuny.edu.
The recent experiments in Hu et al. (Am J Physiol Heart Circ Physiol 279: H1724-H1736, 2000) and Adamson et al. (J Physiol 557: 889-907, 2004) in frog and rat mesentery microvessels have provided strong evidence supporting the Michel-Weinbaum hypothesis for a revised asymmetric Starling principle. These experiments were interpreted using a 3-D mathematical model, Hu et al. (Microvasc Res 58: 281-304, 1999), to describe the coupled water and albumin fluxes across the endothelium with its glycocalyx. This numerical 3-D model converges if the tissue is at uniform concentration or has significant tissue gradients due to tissue loading. However, for most physiological conditions tissue gradients are two to three orders of magnitude smaller than the albumin gradients in the cleft and the numerical model does not converge. A simpler multilayer 1-D analytical model has been developed to describe these conditions. This model is used to extend Michel and Phillips's original 1-D analysis of a matrix layer (J Physiol 388: 421-435, 1987) to include a cleft with a tight junction, to explain the observation of Levick (Exp Physiol 76: 825-857, 1991) that most tissues have an equilibrium tissue concentration close to 0.4 lumen concentration and to explore the role of vesicular transport in achieving this equilibrium. The model predicts the surprising finding of steady-state reabsorption at low pressures, in contrast to the experiments in Michel and Phillips (1987), if a concentration gradient directed toward the tissue is established in the cleft which prevents the concentration from rising behind the glycocalyx.
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