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What is the cardiac cycle?

Class 11 BiologyNCERT chapter: Body Fluids and Circulationइसे हिंदी में पढ़ें

Exam-ready answer

The cardiac cycle is the sequence of events in the heart that is repeated cyclically: systole (contraction) and diastole (relaxation) of both the atria and the ventricles. At 72 beats per minute, one cycle lasts about 0.8 seconds. It includes joint diastole, atrial systole and ventricular systole, followed by ventricular diastole.

Explanation

The cycle begins with all four chambers in joint diastole. The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and the vena cava flows through the left and right atria into the ventricles. The semilunar valves are closed at this stage. The SA node then generates an action potential that makes both atria contract (atrial systole), increasing the flow of blood into the ventricles by about 30 per cent.

The action potential is conducted by the AV node and AV bundle through the ventricular musculature, causing ventricular systole while the atria relax. Rising ventricular pressure closes the tricuspid and bicuspid valves, preventing backflow into the atria; this produces the first heart sound, 'lub'. As pressure rises further, the semilunar valves open and blood flows into the pulmonary artery and the aorta.

The ventricles then relax (ventricular diastole), and their pressure falls. The semilunar valves close to stop backflow, producing the second heart sound, 'dub'. When ventricular pressure falls further, the AV valves open, blood again fills the ventricles, and the heart returns to joint diastole. Each ventricle pumps about 70 mL per beat (stroke volume), so cardiac output is about 5 litres per minute.

Points to remember

  1. Cardiac cycle: one sequence of systole and diastole of the atria and ventricles; duration about 0.8 s.
  2. Heart rate is about 72 beats per minute, so about 72 cardiac cycles occur per minute.
  3. The SA node is the pacemaker; the impulse spreads through the AV node, bundle of His and Purkinje fibres.
  4. First heart sound (lub): closure of tricuspid and bicuspid valves; second (dub): closure of semilunar valves.
  5. Cardiac output = stroke volume × heart rate ≈ 70 mL × 72 ≈ 5 litres per minute.

Exam tip

Write the events in the correct order (joint diastole → atrial systole → ventricular systole → ventricular diastole) and link each valve closure to its heart sound.

Questions students also ask

What is stroke volume?
Stroke volume is the volume of blood pumped out by each ventricle during one cardiac cycle. In a healthy adult it is about 70 mL. Multiplying stroke volume by heart rate gives the cardiac output, which is about 5 litres per minute.
What causes the 'lub' and 'dub' sounds?
'Lub' is the first heart sound, produced when the tricuspid and bicuspid valves close at the start of ventricular systole. 'Dub' is the second heart sound, produced when the semilunar valves close at the start of ventricular diastole.
What is joint diastole?
Joint diastole is the phase when all four chambers of the heart are relaxed. The AV valves are open and the semilunar valves are closed, so blood returning through the vena cava and pulmonary veins fills the atria and passes into the ventricles.

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