The cardiac cycle

Before going in details of cardiac cycle following vocabulary should be clear:

    1. A ventricular systole: when ventricles
    2. A ventricular diastole: when ventricles relax
    3. Bicuspid valves/Mitral valves: valve between left atrium and left
    4. Tricuspid valves: valve between right atrium and right ventricle

(Bicuspid valves or Mitral valves/ Tricuspid valves are also called atrioventricular valves)

    1. Semi-lunar valves: valves between
    • Aortic valve(Left ventricle and aorta)
    • Pulmonary valve ( Right ventricle and pulmonary arch/artery )
    1. Lub sound:
    • Sound 1.
    • Beginning of systole.
    • Tricuspid and Mitral valves close.
    1. Dub sound:
    • sound 2
    • End of systole.
    • Beginning of diastole.
    • Aortic and Pulmonary valves close
    1. SA node/Sinoatrial node:
    • Natural Pace-maker
    • Located in right atrium
    • Electrical activity initiates.
    • Causes the contraction of atria.
    1. AV node/Atrioventricular node:
    • Located between atria and ventricles(anterior-inferior end of the interatrial septum)
    • Act like a gate that slows the electrical signals before enter into
    • This delay gives time to atria to atria before
    1. His-bundles:
    • Cells originate at AV node and enters interventricular septum.
    • Divides into right and left bundle branches.
    • Moves around the tip of ventricular chambers and travel back toward atria along outer walls.
    • Important part of electrical conduction system.
    • Transmits impulses from AV node to the
    1. Purkinje Network:
    • Network of fibers receive electrical signals from AV node.
    • Causes the contraction of both ventricles simultaneously by stimulating ventricular myocardium (cardiac muscle tissue).
    1. Total Cardiac Cycle time is : 8 second
    • Atrial systole : 1 second
    • Atrial diastole: 7 second
    • Ventricular systole: 3 second
    • Ventricular diastole: 5 second
    • Joint diastole: 4 second


Cardiac cycle occurs in the following steps:

    1. At the beginning of cardiac cycle both atria and ventricles are relaxed.
    2. The right atrium receives blood from vena cavae.
    3. The left atrium receives blood from pulmonary veins.
    4. The electrical impulse create from SA node.
    5. SA node causes atria to contract. Atrial systole begins. This is also called atrial depolarization.(P-wave)

         6.With atrial depolarization complete, the impulse is delayed at the AV node. This delay gives the atria time to contract before the ventricles.(P-R interval)

          7.Ventricular depolarization occurs. (QRS-complex) In this phase following events occurs:

      • Atrial repolarization (relax).
      • Ventricles depolarization (contract).
      • Pressure increased in ventricles.
      • AV valves closed. (Bicuspid and tricuspid valves).
      • This produces LUB sound.

        8.Pressure in the ventricles become higher.(ventricular depolarization) Pressure in left ventricle is higher than right  ventricle. This results semilunar valves of aortic and pulmonary arches forced open. Blood ejects from left ventricle to aortic arch and right ventricle to pulmonary arch.

        9.Ventricular repolarization begins at apex (ventricular diastole).(T-Wave) Pressure drops in ventricles. Semi-lunar valves close. This produces a soft sound DUB.

       10.Ventricular repolarization completed.

     11.Now Joint diastole occurs which means that:

      • both atria and ventricles are relaxed for 0.4 seconds
      • during which right auricle receive deoxygenated blood from vena cava and left auricle receive oxygenated from lungs through pulmonary vein.
      • Bicuspid and tricuspid valves open.
      • Chambers relax and blood fills ventricles passively.

Summary of the Cardiac Cycle:


ventricular systole

ventricular diastole

muscles of ventricles



volume of ventricles







· semilunar valves open

· cuspid closed

·    semilunar valves close

·    cuspid open

direction of blood flow

·  left ventricle to Aorta

·  right ventricle to pulmonary artery

atria to ventricle

pressure in Aorta

120 mm Hg

80 mm Hg

Test yourself:

    • Define a cardiac cycle.
    • In how many phases cardiac cycle takes place?
    • What is diastole?
    • What is systole?
    • How much total time is taken by cardiac cycle to take place?
    • Name another name for bicuspid valves.
    • What is the between bicuspid and tricuspid valves?
    • What is the function of semi lunar valve?
    • What is the first sound(S1) of heart beat called?
    • When does the second sound(S2) of heart beat produce?
    • What do you mean by depolarization and repolarization of ventricles?
    • What is the main function of SA node?
    • What does NA node stand for?
    • What is the difference between His-bundles and purkinje fibers?
    • Name the most muscular chamber of the human heart.
    • What do you mean by joint diastole?
    • What is the total time for joint diastole?
    • What does ECG stand for?
    • What does QRS-complex shows on ECG?
    • What is the difference between interventricular and interatrial septum?