Mechanical, Structural, and Physiologic Differences in Human Elastic and Muscular Arteries of Different Ages: Comparison of the Thoracic Aorta to the Superficial Femoral Artery

Abstract

Elastic and muscular arteries differ in structure, function, and mechanical properties, and may adapt differently to aging. We compared the descending thoracic aortas (TA) and the superficial femoral arteries (SFA) of 27 tissue donors (average 41±18 years, range 13-73 years) using planar biaxial testing, constitutive modeling, and bidirectional histology. Both TAs and SFAs increased in size with age, with the outer radius increasing more than the inner radius, but the TAs thickened 6-fold and widened 3-fold faster than the SFAs. The circumferential opening angle did not change in the TA, but increased 2.4-fold in the SFA. Young TAs were relatively isotropic, but the anisotropy increased with age due to longitudinal stiffening. SFAs were 51% more compliant longitudinally irrespective of age. Older TAs and SFAs were stiffer, but the SFA stiffened 5.6-fold faster circumferentially than the TA. Physiologic stresses decreased with age in both arteries, with greater changes occurring longitudinally. TAs had larger circumferential, but smaller longitudinal stresses than the SFAs, larger cardiac cycle stretch, 36% lower circumferential stiffness, and 8-fold more elastic energy available for pulsation. TAs contained elastin sheets separated by smooth muscle cells (SMCs), collagen, and glycosaminoglycans, while the SFAs had SMCs, collagen, and longitudinal elastic fibers. With age, densities of elastin and SMCs decreased, collagen remained constant due to medial thickening, and the glycosaminoglycans increased. Elastic and muscular arteries demonstrate different morphological, mechanical, physiologic, and structural characteristics and adapt differently to aging. While the aortas remodel to preserve the Windkessel function, the SFAs maintain higher longitudinal compliance.