Segments of dog carotid artery were studied in vitro at four longitudinal extension ratios, lambda z = 1.2, 1.4, 1.6, and 1.8, in randomized order. At each length, the pressure was elevated in steps to 200 mmHg or until the vessels buckled. Vessels were studied under control conditions and after treatment with moderate doses of degradative enzymes: 80 U/ml elastase for 90 min or 640 U/ml collagenase for 120 min. These doses were selected, following pilot studies, to degrade vessels but not to destroy them. Treatment with elastase (n = 24) reduced both longitudinal and circumferential stresses at all vessel lengths. Circumferential stress was reduced at pressures greater than 15 mmHg, the magnitude of effect increasing with both longitudinal and circumferential deformations. Longitudinal stress was reduced by a constant amount, irrespective of vessel length. Treatment with collagenase (n = 24) reduced circumferential stress when the vessels were distended by at least 60 mmHg; it did not reduce longitudinal stress. These data suggest that in intact cylindrical segments of dog carotid artery, subjected to physiological levels of strain, elastin bears a portion of both circumferential and longitudinal loads, whereas collagen bears a portion of only circumferential loads.

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SUMMARYLagrangian hydrodynamics of strength-free materials continues to present open issues, even in one dimension. We focus on the problem of closing a system of equations for a two-material cell under the assumption of a single velocity model. There are several existing models and approaches, each possessing different levels of fidelity to the underlying physics and each exhibiting unique features in the computed solutions. We consider three models that take different approaches to breaking the assumption of instantaneous pressure equilibration in the mixed-material cell. The first of these is the well-known method of Tipton, in which a viscosity-like pressure relaxation term is coupled with an otherwise isentropic pressure update to obtain closed-form expressions for the materials' volume fractions and corresponding sub-cell pressures. The second is the physics-inspired, geometry-based pressure relaxation model of Kamm and Shashkov, which is based on an optimization procedure that uses a local, exact Riemann problem. The third model is the unique aspect of this paper, inspired by the work of Delov and Sadchikov and Goncharov and Yanilkin. This sub-scale dynamics approach is motivated by the linearized Riemann problem to initialize volume fraction changes, which are then modified, via the materials' SIEs, to drive the mixed cell toward pressure equilibrium. Each of these approaches is packaged in the framework of a two-step time integration scheme. We compare these multi-material pressure relaxation models, together with corresponding pure-material calculations, on idealized, two-material problems with either ideal-gas or stiffened-gas equations of state.

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Experiments were performed to indentify the series elastic component (SEC) in intact dog carotid artery held at in situ length. The vessels were studied during excitation of the muscle with norepinephrine and after metabolic poisoning with potassium cyanide and sodium iodoacetate. Static circumferential stress-strain curves and stress-quick-release stiffness curves were examined to evaluate Maxwell and Voigt model elements. The vessels were studied at 33, 36, and 39 degrees C. Temperature variations altered active stress, but did not alter connective tissue properties or the Maxwell SEC stiffness. The Voigt model SEC stiffness was altered, but this was secondary to changes in active stress. Thus, most of the SEC is separate from the contractile apparatus. Other vessels were treated with elastase, collagenase, or hyaluronidase to digest the connective tissue components of the wall. Hyaluronidase had no effect on mechanics. Elastase and collagenase altered connective tissue properties, but only elastase unequivocally altered SEC stiffness. This analysis indicated 1) that the carotid artery wall is better represented by a Maxwell model than a Voigt model, and 2) that the SEC in intact carotid artery is primarily elastin.

Longitudinal retraction of carotid arteries, was examined in 105 neonatal puppies as a measure of longitudinal traction. Percent vessel retraction increased linearly with age. This was attributed to stretching of the vessels by growth and to changes in connective tissue composition. The mechanical significance of artery retraction was discussed.