The beginning of systole


The atrioventricular (AV) valves close at the beginning of this phase.

Electrically, ventricular systole is defined as the interval between the QRS complex and the end of the T wave (the Q-T interval).

Mechanically, the isovolumic phase of ventricular systole is defined as the interval between the closing of the AV valves and the opening of the semilunar valves (aortic and pulmonary valves).

Pressures & Volume:

The AV valves close when the pressure in the ventricles (red) exceeds the pressure in the atria (yellow). As the ventricles contract isovolumetrically -- their volume does not change (white) -- the pressure inside increases, approaching the pressure in the aorta and pulmonary arteries (green).


The electrical impulse propagates from the AV node through the His bundle and Purkinje system to allow the ventricles to contract from the apex of the heart towards the base.

The QRS complex is due to ventricular depolarization, and it marks the beginning of ventricular systole. It is so large that it masks the underlying atrial repolarization signal.

Heart sounds:

The first heart sound (S1, "lub") is due to the closing AV valves and associated blood turbulence.

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