Forward coronary flow normally seen in systole is the result of both forward and concealed back flow.

Normally systolic coronary blood flow is almost entirely forward. As perfusion pressure was lowered through the autoregulatory range in open-chest dogs, net systolic back flow appeared at approximately 70 mm Hg. Imposing a series resistance (Rs), which impedes both forward and back flow, abolished this reverse flow and resulted in net forward systolic flow. Thus we conclude that under normal perfusion conditions the pattern of net forward systolic flow contains a substantial reverse flow component.