What is the primary purpose of the cardiac cycle?

The primary purpose of the cardiac cycle is to ensure the continuous and efficient pumping of blood throughout the body. This rhythmic process maintains blood pressure, delivers oxygen and essential nutrients to all tissues and organs, and removes metabolic waste products like carbon dioxide. Each cycle ensures that the heart fills adequately with blood during diastole and then ejects a sufficient volume during systole to meet the body's varying physiological demands, thus sustaining life.

How do heart valves function during the cardiac cycle?

Heart valves act as one-way gates, opening and closing passively in response to pressure gradients across them. During ventricular filling, AV valves (tricuspid and bicuspid) open when atrial pressure exceeds ventricular pressure. During ventricular contraction, these valves close to prevent backflow into the atria. Conversely, semilunar valves (aortic and pulmonary) open when ventricular pressure exceeds arterial pressure, allowing blood ejection, and then close when ventricular pressure drops below arterial pressure, preventing backflow into the ventricles. Their precise timing is crucial for unidirectional blood flow.

What causes the 'lub-dub' heart sounds?

The characteristic 'lub-dub' sounds of the heart, known as S1 and S2, are primarily caused by the closure of heart valves. The 'lub' (S1) sound occurs at the beginning of ventricular systole and is produced by the simultaneous closure of the atrioventricular (tricuspid and bicuspid) valves. This prevents blood from flowing back into the atria. The 'dub' (S2) sound occurs at the beginning of ventricular diastole and is caused by the simultaneous closure of the semilunar (aortic and pulmonary) valves, preventing blood from flowing back into the ventricles from the great arteries.

What is the significance of End-Diastolic Volume (EDV) and End-Systolic Volume (ESV)?

EDV is the maximum volume of blood in the ventricles at the end of diastole, just before ventricular contraction. It represents the preload on the heart. ESV is the minimum volume of blood remaining in the ventricles at the end of systole, after ejection. The difference between EDV and ESV gives the stroke volume (SV), which is the amount of blood ejected per beat. These volumes are critical indicators of cardiac function, influencing cardiac output and reflecting the heart's pumping efficiency.

How does the cardiac cycle relate to the Electrocardiogram (ECG)?

The ECG records the electrical activity of the heart, which precedes and triggers the mechanical events of the cardiac cycle. The P wave on the ECG represents atrial depolarization, which initiates atrial systole. The QRS complex signifies ventricular depolarization, immediately preceding ventricular systole (isovolumetric contraction). Finally, the T wave indicates ventricular repolarization, which marks the end of ventricular systole and the beginning of ventricular diastole (isovolumetric relaxation). This precise electrical-mechanical coupling is fundamental to cardiac function.

Why is the joint diastole phase the longest in the cardiac cycle?

Joint diastole, where both atria and ventricles are relaxed, is the longest phase (approximately 0.4 seconds at rest) because it allows ample time for the ventricles to fill passively with blood. This passive filling accounts for about 70-80% of the ventricular volume before atrial contraction. A longer diastole ensures adequate ventricular filling, especially at lower heart rates, which is crucial for maintaining sufficient cardiac output. It also provides the heart muscle with a longer period of rest and perfusion.

Cardiac Cycle

Biology

NEET UG

Version 1Updated 22 Mar 2026

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The cardiac cycle represents the complete sequence of events occurring in the heart from the beginning of one heartbeat to the beginning of the next. It encompasses a precise, coordinated series of mechanical and electrical activities, including contraction (systole) and relaxation (diastole) of the atria and ventricles, resulting in the efficient pumping of blood throughout the systemic and pulmo…

Quick Summary

The cardiac cycle is the sequence of events in the heart from one heartbeat to the next, typically lasting 0.8 seconds at a resting heart rate of 75 bpm. It comprises two main phases: diastole (relaxation and filling) and systole (contraction and ejection).

The cycle begins with joint diastole, where all chambers relax and ventricles passively fill. This is followed by atrial systole, where atria contract to push the remaining blood into the ventricles, reaching the End-Diastolic Volume (EDV).

Next is ventricular systole, starting with isovolumetric contraction (all valves closed, pressure rising, S1 sound from AV valve closure), then ventricular ejection (semilunar valves open, blood pumped out, leaving End-Systolic Volume (ESV)).

Finally, ventricular diastole begins with isovolumetric relaxation (all valves closed, pressure falling, S2 sound from semilunar valve closure), followed by rapid and reduced ventricular filling, leading back to joint diastole.

Pressure gradients drive blood flow and valve actions, ensuring unidirectional circulation. The ECG correlates electrical events (P wave, QRS complex, T wave) with these mechanical phases.

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Cardiac Cycle Duration: —

0.8,\text{s}

(at

75,\text{bpm}

)

Phases: — Atrial Systole (

0.1,\text{s}

), Ventricular Systole (

0.3,\text{s}

), Joint Diastole (

0.4,\text{s}

)

Ventricular Systole Sub-phases: — Isovolumetric Contraction (

0.05,\text{s}

), Ventricular Ejection (

0.25,\text{s}

)

Ventricular Diastole Sub-phases: — Isovolumetric Relaxation (

0.08,\text{s}

), Rapid Filling (

0.11,\text{s}

), Diastasis (

0.21,\text{s}

)

Heart Sounds:

- S1 ('lub'): Closure of AV valves (Tricuspid & Bicuspid) - marks start of ventricular systole. - S2 ('dub'): Closure of Semilunar valves (Aortic & Pulmonary) - marks start of ventricular diastole.

ECG Correlation:

- P wave: Atrial depolarization (precedes atrial systole). - QRS complex: Ventricular depolarization (precedes ventricular systole). - T wave: Ventricular repolarization (during ventricular ejection, precedes ventricular diastole).

Volumes:

- EDV (End-Diastolic Volume): Max ventricular volume (approx. $120-130,\text{mL}$ ). - ESV (End-Systolic Volume): Min ventricular volume (approx. $50-60,\text{mL}$ ). - SV (Stroke Volume): $EDV - ESV$ (approx. $70,\text{mL}$ ).

Valve Actions: — Open/close passively due to pressure gradients.

To remember the sequence of events in the cardiac cycle, think: All Valves In Every Room Really Dance.

Atrial Systole

Ventricular Isovolumetric Contraction

Ejection (Ventricular)

Relaxation (Ventricular Isovolumetric)

Rapid Filling (Ventricular)

Diastasis (Reduced Filling)