Changes in smooth muscle tone influence characteristic impedance of the aorta.

Experiments were performed on anesthetized dogs to quantitate the effects of changes in aortic smooth muscle tone on the passive pressure dependence of the aortic characteristic impedance (Zc). Zc was calculated from the diastolic pressure-diameter relationship in a wide pressure range induced by forced slow oscillations in pressure. Changes in aortic smooth muscle activity were induced by hemorrhage (-15% of the estimated blood volume), by volume expansion (+15 and +30%), and by alpha-adrenergic blockade. Curves relating Zc to pressure showed a minimum in the physiological pressure range and were well described by parabolas under all conditions. Under control conditions, the minimum Zc pressure was located close to the mean arterial pressure. Both levels of volume expansion as well as alpha-blockade reduced Zc at all pressure levels. Hemorrhage increased Zc at each pressure level and caused the minimum in the Zc vs. pressure curves to shift to higher values. The observed changes in Zc may play an important role in the hemodynamic adjustments to hemorrhage and to volume expansion.