As a typical muscular conduit artery, the renal artery manifests an active response when the vascular smooth muscle is stimulated to contract. To present there are few experimental data [1] and no constitutive formulation of the renal arterial tissue that accounts for the active stress developed when the smooth muscle cells (SMCs) are stimulated to contract. Most studies identify the circumferential and axial active stresses by processing the in-vitro experimentally recorded change in diameter and axial force in a tubular arterial specimen inflated by an internal pressure, kept at constant axial stretch ratio, and with the SMCs stimulated to relax or contract. It is assumed that the stress borne by the extracellular matrix, termed as passive stress, and the active stress generated by the stimulated SMCs are additive. The directions of the active stresses and functional dependence on the strain measures are often postulated on the basis of histology, biophysics of SMC contraction, and an a priori guess, rather than on recordable mechanical information. The only exception is a recently proposed approach that allows decoupling of the passive and active response and identifying the arguments and the analytical form of the active stress directly from experimental data [2]. In this study, we follow this approach and determine the active stress in the porcine renal artery.

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