What is the cardiac cycle?

The cardiac cycle is the complete sequence of events during one heartbeat, lasting about 0.8 seconds at 75 bpm. It includes three phases: Joint Diastole (all chambers relax and fill, 0.4s), Atrial Systole (atria contract to complete filling, ~0.1s), and Ventricular Systole (ventricles contract and pump blood out, ~0.3s). The SA node initiates each cycle.

What happens during Joint Diastole?

During joint diastole, all four chambers of the heart relax. The AV valves (tricuspid and mitral) open because atrial pressure exceeds ventricular pressure. Blood flows passively from the great veins (SVC, IVC, pulmonary veins) through the atria into the ventricles. About 70-80% of ventricular filling occurs during this passive phase.

What is the atrial kick?

The atrial kick is the active contraction of the atria during Atrial Systole (the second phase of the cardiac cycle). It pushes the remaining 20-30% of blood into the ventricles, completing ventricular filling to the End-Diastolic Volume (~130 ml). The AV valves remain open during this phase. The atrial kick is initiated by the P-wave on the ECG.

When do the AV valves close and why?

The AV valves (tricuspid and mitral) close at the beginning of ventricular systole, when ventricular pressure rises above atrial pressure. Their closure produces the S1 heart sound ('Lub'). They remain closed throughout ventricular systole to prevent backflow of blood from the contracting ventricles into the atria.

What are the heart sounds S1 and S2?

S1 ('Lub') is produced by the closure of the two AV valves (tricuspid and mitral) at the beginning of ventricular systole. S2 ('Dub') is produced by the closure of the two semilunar valves (pulmonary and aortic) at the beginning of diastole, after ventricular systole. So the cycle sounds like 'Lub...Dub' — S1 marks systole onset, S2 marks diastole onset.

How long does one cardiac cycle last?

One cardiac cycle lasts approximately 0.8 seconds at a resting heart rate of 75 bpm. This breaks down as: Joint Diastole ~0.4s, Atrial Systole ~0.1s, Ventricular Systole ~0.3s. At higher heart rates (e.g., exercise), the diastolic phase is shortened first.

What is end-diastolic volume vs end-systolic volume?

End-Diastolic Volume (EDV) is the volume of blood in each ventricle at the end of filling (after atrial systole), approximately 130 ml. End-Systolic Volume (ESV) is the volume remaining after ventricular contraction, approximately 50 ml. The difference is the Stroke Volume: SV = EDV − ESV = 80 ml. Ejection fraction = SV/EDV = 80/130 ≈ 62%.

Is the cardiac cycle a NEET topic?

Yes. The cardiac cycle is a high-weightage topic in the Class 11 chapter 'Body Fluids and Circulation.' NEET frequently tests which valves open/close in each phase, heart sounds S1 and S2, EDV and ESV values, stroke volume calculation, and the distinction between pulmonary and systemic circulation pressures. It appears in almost every NEET paper.

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Cardiac Cycle3D Lab · Medium

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Cardiac Cycle

Human PhysiologyMediumNEET: 2-3 Q3D

Zoology lab

Cardiac Cycle — Chambers, Valves & Blood Flow

Circulatory System

Explore the interactive model.

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switch phases to see events · click parts to learn · drag to rotate

How to use this lab

1Firing/phase slider drag karo — diastole → systole, valves khulte-band dekho.
2“Cut open” dabao — chambers + blood flow (deoxygenated & oxygenated path).
3🏷 Labels on rakho; Wiggers diagram heart ke saath chalta hai.

About this lab

Watch the heart beat in 3D. Switch between Joint Diastole, Atrial Systole, and Ventricular Systole to see how all four chambers contract, valves open and close, and blood flows through the heart in real time.

What you're seeing

The cardiac cycle is the sequence of events during one heartbeat (~0.8 seconds at 75 bpm). It has three main phases: Joint Diastole (all chambers relax, 70-80% passive filling), Atrial Systole (atria contract, 'atrial kick' adds 20-30%), and Ventricular Systole (ventricles contract, blood ejected into arteries). The SA node (pacemaker) initiates each cycle with an electrical impulse.

Key NEET facts

Cardiac cycle duration: ~0.8 s (at 75 bpm)
Joint Diastole 0.4s + Atrial Systole 0.1s + Ventricular Systole 0.3s = 0.8s
End-Diastolic Volume (EDV): ~130 ml per ventricle
volume of blood in ventricle at end of filling (after atrial kick)
End-Systolic Volume (ESV): ~50 ml per ventricle
volume remaining after ventricular ejection
Stroke Volume (SV): SV = EDV − ESV = 80 ml
volume pumped per beat per ventricle. Ejection fraction = 80/130 ≈ 62%
Heart sounds: S1 'Lub' = AV valves close; S2 'Dub' = SL valves close
S1 marks start of ventricular systole; S2 marks start of diastole

Memory trick

JAB-ASK-VLC: Joint diastole = Juices flow, Atria fill, Blood passive. Atrial Systole = Atria Squeeze, Kick. Ventricular Systole = Ventricles contract, Lub sound, Cardiac output. S1 = AV valves shut (ventricular systole starts). S2 = SL valves shut (diastole starts).

Practice

Test Yourself

During which phase of the cardiac cycle do the semilunar valves open?

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