A number of factors influence blood flow into the heart in addition to atrial contraction, such as: passive blood flow, active ventricular relaxation, upward movement of the ventricle, pericardial restraint, and ventricular compliance.
During ventricular relaxation the ventricles move upward, facilitating blood flow from the atria into the ventricles. The pericardium, a tough connective tissue sac, limits ventricular filling, as does also the high level of passive tension found near Lmax in the length-tension diagram. Conditions which diminish ventricular compliance (myocardial infarction, cardiomyopathy, pericardial tamponade), limit ventricular filling.
In the past, emphasis has been placed on contraction as opposed to relaxation. Clearly the pump must be filled before it can eject. During the past decade the essential role of relaxation has begun to be realized. Death from heart failure often occurs as the result of the inability of the ventricle to relax, thus fill. Studies have found that intraventricular pressure continues to fall during rapid filling and negative intraventricular pressures have been reported, suggesting that the ventricle "sucks" blood from the atria, veins, etc.
Because the heart is doing work on the blood after contraction ceases, this has caused some physiologists to espouse alternate views of the cardiac cycle, where systole is re-defined as the period during which the heart is doing work, in whatever form. In this view, systole also includes isovolumic relaxation and rapid filling.
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