One cardiac cycle includes contraction and relaxation of atria and ventricles. During joint diastole, all chambers are relaxed and blood flows into atria and through open atrioventricular valves toward ventricles. Atrial systole adds a final contribution to ventricular filling. Ventricular systole then raises pressure, closes AV valves, opens semilunar valves and ejects blood.

When ventricles relax, their pressure falls, semilunar valves close and filling resumes. At a classroom resting heart rate near 72–75 beats per minute, a cycle lasts roughly 0.8 second. The exact length depends on rate; do not assume every component scales identically at high rate. The electrical activation and mechanical phases are related but not identical events.

Worked example: At 75 beats/min, cycle duration is 60/75 = 0.8 s.

NCERT trap: Atrial contraction precedes ventricular contraction; both do not begin from the same ECG wave.

NEET method: Start with joint diastole, then atrial systole, ventricular systole and relaxation.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Body Fluids and Circulation

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