In this article, we present 1.) a thorough analysis of the anisotropic, or directionally-dependent, mechanical properties of atrioventricular, i.e. mitral and tricuspid, heart valve leaflets, 2.) an examination of the effects of biaxial testing parameters such as loading-rate, testing temperature, and animal donor species on the recorded tissue behaviors, and 3.) a structural analysis of the heart valves, including quantifications of distributions of the valvular chordae tendineae and histological approaches to leaflet-specific microstructural and layer composition quantification. The aforementioned analyses, and the mechanical response data provided, will help to both improve cardiovascular computational modeling efforts and help researchers better understand the biomechanics of heart valve tissue.