Date of Award
Doctor of Philosophy
The design and pathology of arteries are predictable by principles of fluid dynamics and intramural forces and stresses to which the arterial wall is subjected. The angular geometry of bifurcations has been predicted by imposing static equilibrium of longitudinal tensile forces of parent and daughter arteries. This mathematical model predicts that the larger branch makes a smaller angle with the direction of the parent artery. Cerebral aneurysms are proposed to result from an unstable equilibrium which causes a dynamic cycling of the opening angle and ultimately disruption of the arterial wall. Hemodynamics and wall properties of bifurcations have been combined with a hypothesis that bifurcations are designed to minimize impedance mismatching between parent and daughter vessels and I have subsequently shown with a mathematical model that reflections cannot be minimized without changes in elastic properties through bifurcations. Predictions of the mathematical model are qualitatively consistent with the known decrease in Young's moduli from proximal to peripheral arteries though a quantitative comparison has yet to be made. Experimental measurements of vessel calibre and wall thicknesses and wall composition of the tunica intima and media in 19 normotensive and 29 hypertensive human cadavers have shown that the arterial wall thickens in response to hypertension in order to conserve intramural stresses and that the composition of the arterial wall varies with vessel calibre. Circumferential and longitudinal intramural stresses increased linearly with vessel calibre but the regression lines did not differ significantly between normotensives and hypertensives. No qualitative difference was found between the relative medial composition of hypertensives and normotensives. The relative collagen composition of the intima was increased and the relative elastin and smooth muscle compositions were decreased with hypertension whereas the absolute amounts of all components in the intima were increased with hypertension. Since most hypertensive cadavers received antihypertensive therapy I conclude that the lack of difference in composition in the tunica media between normotensives and hypertensives may be secondary to a beneficial bias of treatment whereas composition changes in the intima are not reversible. Further prospective studies are warranted in animal models to validate the conclusions of this retrospective study in humans.
Brown, Norman Neil, "Biophysical Design Of Human Arteries" (1994). Digitized Theses. 2345.