Page 9, Cardiac Cycle, Dr. D. Penney
There are a number of factors which influence blood flow into the heart (Table 1.02). Right atrial filling is dependent upon: the skeletal muscle pump, respiratory movements, venous tone (state of smooth muscle contraction), posture (supine, standing upright), the blood volume, and the downward movement of the ventricle during contraction. Ventricular filling in turn is dependent upon: atrial contraction, passive blood flow, active ventricular relaxation, upward movement of the ventricle, pericardial restraint, and ventricular compliance. The volume of the whole heart remains relatively constant throughout the cardiac cycle; as the ventricles contract and decrease in volume, blood is pulled into the atria increasing their volume - a push-pull arrangement.
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, as seen in the length-tension diagram. Conditions which diminish ventricular compliance (myocardial infarction, cardiomyopathy, pericardial tamponade), limit ventricular filling.
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