The mechanical properties of arteries are an important determinant of hemodynamics. The aorta, pulmonary artery, and large distributing arteries are distended rapidly during ventricular ejection, transiently accommodating 50% or more of the stroke volume (331, 358, 411). These vessels then retract during diastole. Because of these dimensional changes, the viscoelastic properties of the walls of these large vessels are a factor determining instantaneous arterial pressure. The large arteries also transmit the pressure pulse and are the chief site of vascular impedance, ie, the dynamic resistance to the oscillatory components of blood flow. Finally, certain baroreceptor areas of the arterial tree monitor blood pressure by distention,