The microstructure of the collagen sheath or weave surrounding a myocyte and the collagen struts interconnecting neighboring myocytes is incorporated into a fluid-fiber-collagen continuum description of the myocardium. The sheaths contribute to anisotropic elasticity, whereas the struts contribute to an isotropic component. Elastic moduli of the composite myocyte-sheath complex and the strut matrix are estimated from existing passive biaxial loading data from sheets of canine myocardium. The contribution of the sheath to the elasticity of the myocyte-sheath complex is critically dependent on the helical pitch angle. Calculations for a cylindrical model of the left ventricle using both a fluid-fiber and fluid-fiber-collagen stress tensor show that the collagen strut matrix tends to limit muscle fiber lengthening; increase myocardial tissue pressure during systole, with endocardial tissue pressure exceeding left ventricular pressure; decrease tissue pressure during diastole, and thus facilitate myocardial blood flow; and aid filling during ventricular relaxation by providing a suction effect that relies on a release of stored elastic energy from the previous contraction. Calculations show that this energy is stored mostly in the collagen struts
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