Most of the coronary blood flow to the left ventricular myocardium occurs during diastole. Thus the contracting heart obstructs its own blood supply. The systolic compressive force has 2 components. There is LV systolic intracavitary pressure and vascular narrowing caused by contraction of the heart.
Because compressive forces exerted by the RV are ordinarily far smaller than those of LV, ventricular perfusion is reduced, but not interrupted during systole.