Physical Factors Which Influence Vascular Caliber and Blood Flow

Abstract

1. The radii of blood vessels constitute their major biological functions. 2. The radii of blood vessels are abnormally reduced in most forms of hypertension. 3. The radii of blood vessels are determined by distending pressure and by the geometry and stiffness of the wall material. The latter two factors determine the vessel tone. Thus, tone is defined by those factors which determine vessel radius with respect to distending pressure. 4. It is now possible to measure simultaneously the distending pressure and the factors which determine tone, as well as the radius of vessels. Thus, it is possible to quantitate the factors which determine vascular tone: elastic stiffness (E), viscous stiffness (R), radius (r), and wall thickness (8). 5. These factors have been studied in intact dogs, and the tone of the major vessels of the vascular tree has been mapped out. 6. As the vessel wall itself becomes stiffened, compensatory changes may occur in the geometry. As the vessel dilates and thins, the r/8 ratio enlarges, and the vessel becomes relatively more distensible. 7. Little is known about the changes within the vessel wall which are associated with the mechanical properties of the wall, such as smooth muscle, connective tissue, water and electrolyte concentrations and distributions, the nonhomogenous aspects of the tissue components, or the extent of isovolumetric considerations that are applicable. 8. One of the most challenging aspects of cardiovascular research is to define appropriately and measure the factors mentioned above. They certainly represent part of the key to understanding the etiology and development of hypertension.