Explain why ventricular contraction[systole] and the lub sound occur immediately after the QRS complex.?

Answer 1

The QRS complex is the ECG representation of the cardiac electrical impulse travelling beyond the AV node, down the purkinje fibres and to trigger the ventricle.

This is why the ventricular contraction occurs immediately after the QRS complex.

The "lub" sound, or first heart sound (S1) is caused by the closure of the atrioventricular valve. Strictly speaking, it is the sound caused by the cessation of the column of fluid passing through the AV valve. This occurs during the beginning of systole as the ventricular pressure exceeds the atrial pressure and tends to force fluid backwards from ventricle to atrium - only to be stopped by the AV valve.

Hence the S1 occurs right at the beginning of systole, ie immediately after the QRS complex.

For a pictorial representation - see the wiki page and the diagram of the cardiac cycle.

Answer 2

The sounds are not really 'caused ' by the contractions. They are caused by the closing of the valves. So the first sound (we call it S1) is the closing of the mitral valve and the tricuspid valve. The second sound (S2, the one in your question) is the closing of the atrioventricular valve and pulmonary valve. The QRS complex is the electrical activity to cause the ventricular contraction..which would then initiate the second heart sound.

Answer 3

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