Mechanical events and the pressure–volume relationships

Depolarization of cardiac muscle fibres spreads from fibre to fibre throughout the myocardium. In a single fibre, contraction starts just after depolarization and lasts until just after repolarization is complete. The atria contract, completing the filling of the ventricles and thus enhancing their action. In the absence of effective atrial contraction (e.g. atrial fibrillation) cardiac output is decreased on average by 15%. During diastole, when cardiac muscle is relaxed, blood returns to the heart and passes through the atrioventricular (AV) valves into the ventricles. The semilunar valves, between the ventricles and the arteries, are closed as arterial pressure exceeds ventricular pressure. Under normal circumstances, 70% of ventricular filling occurs by late diastole.