Cardiac Cycle: Phases of Heartbeat Explained Step by Step

The cardiac cycle is the full sequence of events in one heartbeat — the filling and emptying of the heart's chambers. At a normal resting rate of about 72 beats per minute, one cycle lasts about 0.8 seconds, and it repeats tirelessly throughout life.

Understanding this cycle matters because blood pressure, heart sounds and ECG readings all come from these phases. When the timing goes wrong, the heart fills poorly or pumps weakly, and doctors trace the fault back to a specific phase.

Key Concepts

• Systole means contraction of heart muscle; diastole means relaxation.
• Joint diastole (about 0.4 s): all four chambers relax and blood flows passively into the ventricles.
• Atrial systole (about 0.1 s): atria contract and push the remaining blood into ventricles.
• Ventricular systole (about 0.3 s): ventricles contract, AV valves shut (first sound, "lub"), semilunar valves open and blood is ejected into aorta and pulmonary artery.
• When ventricles relax, semilunar valves shut, producing the second sound, "dub".
• Stroke volume is the blood pumped per beat, about 70 mL; cardiac output is stroke volume x heart rate, about 5 litres per minute at rest.

Formulas

Cardiac output = Stroke volume x Heart rate. Stroke volume is measured in mL per beat, heart rate in beats per minute, so cardiac output comes out in mL per minute. This relation assumes a steady rhythm and that both ventricles pump equal volumes, which is true in a healthy heart over time.

Worked Example

Take a resting person with heart rate 72 beats per minute and stroke volume 70 mL. Cardiac output = 70 x 72 = 5,040 mL per minute, about 5 litres — matching the total blood volume, so the entire blood supply circulates roughly once every minute. During heavy exercise, heart rate may rise to 150 and stroke volume to 110 mL: output = 110 x 150 = 16,500 mL per minute, over 3 times the resting value, delivering the extra oxygen muscles demand.

Common Misconceptions

• "The heart rests between beats like we rest." Wrong. Diastole is active relaxation and filling; the cycle has no complete pause at normal rates.
• "Atria and ventricles contract together." Wrong. Atria contract first; a short delay lets ventricles fill fully before they contract.
• "Heart sounds come from muscles contracting." Wrong. Lub and dub are sounds of valves closing, not muscle movement.

Real-World Applications

Blood pressure readings like 120/80 mmHg directly reflect ventricular systole and diastole pressures. Athletes train to raise stroke volume so the same output needs fewer beats. Doctors use cycle timing to interpret ECGs and ultrasound scans, and artificial pacemakers copy this sequence when the natural timing fails.

One heartbeat is a timed loop: joint diastole, atrial systole, ventricular systole, then relaxation again. In 0.8 seconds the heart fills, tops up and ejects about 70 mL — and then immediately begins again.