lumped parameter model. Am J Physiol Regulatory Integra-

tive Comp Physiol 282: R611–R622, 2002; 10.1152/ajpregu. 00285.2001.—The dynamic cerebral blood flow response to sudden hypotension during posture change is poorly under-stood. To better understand the cardiovascular response to hypotension, we used a windkessel model with two resistors and a capacitor to reproduce beat-to-beat changes in middle cerebral artery blood flow velocity (transcranial Doppler measurements) in response to arterial pressure changes measured in the finger (Finapres). The resistors represent lumped systemic and peripheral resistances in the cerebral vasculature, whereas the capacitor represents a lumped sys-temic compliance. Ten healthy young subjects were studied during posture change from sitting to standing. Dynamic variations of the peripheral and systemic resistances were extracted from the data on a beat-to-beat basis. The mode