Understanding vascular physiology and pathology requires detailed knowledge of the mechanical properties of blood vessels, and the mechanisms that allow them to remodel and adapt to altered loading conditions. Such adaptation often involves changes in the residual stresses that exist in the tissue even when all external loads are removed, and function to optimize the stresses exerted on cells within the vascular wall. We recently showed that residual stresses can arise from transmurally heterogeneous concentrations of fixed negative charge due to proteoglycan molecules which are preferentially located in the inner layers of the vessel wall. The purpose of this study was to examine how regional differences in proteoglycan heterogeneity contribute to variations in residual stress at different anatomical locations in arteries. This idea was tested by measuring opening angles in rat blood vessels under specific osmotic loading conditions, computing a Donnan opening angle (DOA) due to fixed charge effects, and correlating this with relative proteoglycan distributions obtained by quantitative histological analysis of the arterial vessel wall. Experimentally, the DOA exhibited a second order polynomial dependence on proteoglycan heterogeneity determined from the ratio of media/total wall thickness, with R2=0.724, and a maximum opening angle at a ratio of 48%. This is consistent with a theoretically predicted value of 50%, and supports our hypothesis that proteoglycans play an important role in vascular residual stress. Therefore, modification of proteoglycan content in arterial walls suggests a novel mechanism to regulate in vivo mechanical homeostasis.