1. Total opposition to flow. When flow is steady, impedance is simply the resistance, e.g., the driving pressure per unit flow; when flow is changing, impedance also includes the factors that oppose changes in flow. Thus, deviations of impedance, from simple ohmic resistance because of the effects of capacitance and inductance, become more important in alternating current as the frequency of oscillations increases. In fluid analogies (e.g., pulsatile flow of blood, to-and-fro flow of respiratory gas), impedance depends not only on viscous resistance but also upon compressibility, compliance, inertance, and the frequency of imposed oscillations.