Determinants of vascular hemodynamic characteristics.

This discussion concerns some of the parameters affecting the hemodynamic characteristics of a blood vessel: impedance, flow resistance, and vascular wall stiffness. The parameters of greatest importance are the vessel internal radius (ri) and the elastic modulus of the vascular wall. When the smooth muscle cells in the vascular wall are inactive, the elastic modulus is determined by the combined elastic modulus of the structural components of the connective tissue, primarily elastin and collagen fibers. This combined elastic modulus is found to be proportional to the wall stress. From the proportionality factor and from knowledge of the vessel dimensions in the unpressurized vessel, one can predict the relation of ri to transmural pressure. When a vessel is activated under isometric conditions, the wall force and elastic modulus are increased by the force and elastic modulus of the smooth muscle cells. Under isobaric conditions, however, the effect of activation is to reduce both the ri and the elastic modulus. Pathologic changes resulting either in a stiffening of the connective tissue components, in an increased wall cross-sectional area, which encroaches on the lumen, or in an increased smooth muscle quantity will all cause increases in vascular characteristic impedance, vascular flow resistance, and vascular wall stiffness.