A structure-based model that accurately predicts micro- or macro-mechanical behavior of blood vessels is necessary to understand vascular physiology. Based on recently measured microstructural data, we propose a 3-D microstructural model of coronary adventitia that incorporates the elastin and collagen distributions throughout the wall. The role of ground substance was found to be negligible under physiological axial stretch z=1.3, based on enzyme degradation of glycosaminoglycans in swine coronary adventitia (n=5). The thick collagen bundles of outer adventitia (n=4) were found to be undulated and unengaged at physiological loads, while the inner adventitia consisting of multiple sublayers of entangled fibers that bear the majority of load at higher pressures. The microstructural model was validated against biaxial (inflation and extension) experiments of coronary adventitia (n=5). The model accurately predicted the nonlinear responses of the adventitia, even at high axial force (axial stretch ratio z=1.5). The model also enabled a reliable estimation of material parameters of individual fibers that were physically reasonable. A sensitivity analysis was performed to assess the effect of using mean values of the distributions for fiber orientation and waviness as opposed to the full distributions. The simplified mean analysis affects the fiber stress-strain relation, resulting in incorrect estimation of mechanical parameters, which underscores the need for measurements of fiber distribution for a rigorous analysis of fiber mechanics. The validated structure-based model of coronary adventitia provides a deeper understanding of vascular mechanics in health and can be extended to disease conditions.
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