W.: Variability of the viscoelastic constants along the aortic axis of the dog. Circulation Res

MEASUREMENTS of the elasticity ofisolated aortic specimens are compli-cated by the nonlinear mechanical behavior of such tissues. Nonlinearity occurs in both static and dynamic measurements. Krafka,1-2 in a series of studies on the static elasticity of isolated aortic strips, computed 3 elastic moduli, each at a different elongation. These moduli differed from each other and varied independently under different conditions. Un-der dynamic conditions,3 elastic moduli may be several times greater than static elastic moduli at the same mean elongation. Damping in such specimens appears to be frequency dependent. Nonlinearity of the mechanical properties of blood vessel walls has been attributed to a complex interaction among the various fiber types present.1"0 King,7'8 on the other hand, has interpreted the static mechanical behavior of the aortic wall in terms of modern high polymer theory. Recently, this latter inter-pretation has been extended to include dy-namic data.9-10 The justification for the ap-plication of the polymer theory rests, in part, upon experimental observations that the elas-tic force in isolated aortic specimens is en-tropic over a considerable range of elonga-tion.11 Anomalous thermoelasticity, nonlinear stress-strain curves and long range reversible extensibility are characteristics usually asso-ciated with rubber-like materials. Many bio