It is important to remember that the properties described so far (resistance, turbu­lence, etc.) and their effects on ow apply only to a model with a static pressure– ow relationship. However, the pulsatile nature of the pressure in extra- and intracranial vessels results in a dynamic pressure–ow relationship because these vessels are elastic. The most important dynamic relationship is known as the “Windkessel” effect. The Windkessel model described by Otto Frank [42, 43] denes the hemodynamics of the arterial system in terms of resistance and compli­ance. It describes the aortic pressure decline during diastole but not entirely so dur­ing systole. Impedance was proposed as a third component of the Windkessel model. The heart ejects blood intermittently; however, the ow is more continuous as the blood pressure pulsates because the elasticity of the arterial system acts as a reser­voir of the energy supplied during systole, thus making the system more efcient. This model can then predict the classic shape of the ow waveform seen in a pulsa­tile system [42, 43]. Windkessel is roughly translated from German to English as “air chamber,” drawing the analogy with the air chamber used in re engines in the eighteenth century [44]. CBF waveforms obtained by TCD are consistent with low­resistance ow, which changes in pathological states. The Windkessel model is a lumped model and therefore not appropriate for the evaluation of wave travel, but it is a fair approximation of ventricular afterload.