Downey and Kirk [1975] developed a coronary circulation model assuming that local blood flow in the myocardium is determined by waterfall mechanism. Panel [A] shows the collapsible-wall vessel surrounded by intramyocardial pressure (Pt). Panel [C] shows that coronary blood flow (CBF) is proportional to perfusion pressure (Pa) when Pt=0. Increasing Pt above zero, flow stops when Pt>Pa, since the higher pressure outside of the vessel causes it to collapse along its entire length. When Pt is between Pa and Pv, a partial collapse occurs at the point where pressure within the vessel falls below Pt. Panel (B) shows the electrical analogue of the waterfall model. It consists of a resistor in series with a diode and a battery. The diode-battery combination represents the intramyocardial pressure (the voltage Vt is equivalent to Pt, the voltage Va is equivalent to Pa). Two or more circuits presented in panel [B] can be assembled in parallel, each of them representing a layer of myocardium. The battery voltage is assumed to be proportional to left ventricular pressure (Plv), with a proportionality constant that decreases from lumen to epicardium taking negligible values there. According to Spaan [1981] the presented electrical circuit cannot describe all oscillatory pressure-flow relations observed experimentally. For instance, systolic arterial back-flow that occurs for low inflow pressures cannot be represented using the waterfall model.