Five phases of cardiac cycle & Atrial contraction

1. Atrial contraction (atrial systole). 2. Isovolumetric (ventricular) contraction. 3. Ventricular ejection. 4. Isovolumetric (ventricular) relaxation. 5. Passive ventricular filling. Beginning of cardiac cycle • Prior to atrial contraction: • Atria and ventricles are relaxed. • Atrioventricular (AV) valves are open. • Semilunar (SL) valves are closed. • Atrial depolarization (P wave) triggers atrial contraction. • Causes atrial pressure to rise. • Pushes blood into the ventricles, raising ventricular pressure slightly. • Ventricular excitation (QRS wave) triggers ventricular contraction.

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