Step by Step
1
Diastole — filling
The AV valves are open, and blood flows passively from the atria into the ventricles. Right at the end, atrial systole (the "atrial kick") adds the last 20-30% of filling by actively contracting the atria.
2
Isovolumetric contraction — Lub (S1)
The ventricles begin contracting, and pressure rises rapidly. This pressure rise forces the AV valves to close — producing the first heart sound, S1 ("Lub"). Briefly, both sets of valves (AV and semilunar) are closed.
3
Ventricular ejection
Once ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is forcefully ejected — this ejected volume is called the stroke volume, typically around 70 mL.
4
Isovolumetric relaxation — Dub (S2)
The ventricles relax, and pressure drops rapidly. This causes the semilunar valves to close — producing the second heart sound, S2 ("Dub"). Pressure continues dropping until it falls below atrial pressure, at which point the AV valves open again and the cycle repeats.
Applied Walkthrough
1
During diastole, the AV valves are open and blood flows passively into the ventricles, topped off by the atrial kick.
2
As the ventricles begin contracting, pressure rises sharply, snapping the AV valves shut — this produces the S1 ("Lub") sound, heard through a stethoscope.
3
Once ventricular pressure exceeds the pressure in the arteries, the semilunar valves swing open and blood is ejected out — the stroke volume.
4
As the ventricles then relax, pressure drops and the semilunar valves snap shut, producing the S2 ("Dub") sound — completing one full cardiac cycle, ready to begin filling again.
Exam Application
Exams test whether you can correctly sequence the four phases of the cardiac cycle, and whether you can correctly attribute S1 ("Lub") to AV valve closure and S2 ("Dub") to semilunar valve closure.
⚠ Common Trap
The most common trap is reversing which valve closure produces which heart sound — remember S1 ("Lub") is the AV valves closing at the start of ventricular contraction, and S2 ("Dub") is the semilunar valves closing at the start of ventricular relaxation.
✓ Quick Self-Check
1. What happens during diastole?
The AV valves are open, and blood flows passively (plus the atrial kick) from the atria into the ventricles.
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2. What produces the S1 ("Lub") heart sound?
The AV valves closing, at the start of isovolumetric contraction.
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3. What produces the S2 ("Dub") heart sound?
The semilunar valves closing, at the start of isovolumetric relaxation.
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4. What is stroke volume?
The volume of blood ejected from the ventricle during ventricular ejection, typically around 70 mL.
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5. What does an S3 heart sound (a ventricular gallop) suggest clinically?
Heart failure.
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