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.

Human PhysiologyMediumNEET: 2-3 Q3D

Cardiac Cycle — Chambers, Valves & Blood Flow

Circulatory System

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.

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Cardiac Cycle — Chambers, Valves & Blood Flow

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