Cardiac Cycle

Cardiac Cycle Overview

  • The cardiac cycle consists of alternating contractions and relaxations of the atria and ventricles, causing blood to move from areas of higher pressure to areas of lower pressure.

    • Blood always flows from high pressure to low pressure.

Heart Chamber Contraction and Relaxation

  • Contraction of the heart chambers is called systole.

  • Relaxation of the heart chambers is called diastole.

  • When a chamber of the heart contracts, the pressure inside increases.

  • When a chamber of the heart relaxes, the pressure inside drops.

Duration of a Cardiac Cycle

  • In an average sized healthy adult at rest:

    • One cardiac cycle lasts approximately 0.8 seconds.

    • About half of this time is for systole (contraction) and the other half for diastole (relaxation).

Phases of the Cardiac Cycle

1. Ventricular Filling

  • Occurs after the ventricles have ejected blood.

  • The right ventricle pushes blood into the pulmonary trunk, and the left ventricle pushes blood into the aorta.

  • Ventricles relax, which causes pressure inside ventricles to drop quickly.

  • Atria have higher pressure than ventricles, leading to the opening of the atrioventricular (AV) valves (tricuspid and bicuspid valves).

  • Blood flows from atria to ventricles, during which:

    • The entire heart is relaxed.

    • This phase is called ventricular filling.

2. Atrial Systole

  • Occurs after ventricular filling.

  • Atrial walls contract, contributing an extra volume of blood into the ventricles.

  • This phase lasts about 0.1 seconds.

  • Approximately 25 milliliters of blood is added to the ventricles during this phase.

  • At the end of atrial systole, the ventricles are also done with relaxation, nearing the end of ventricular diastole.

3. Isovolumetric Contraction

  • Following atrial systole, the atrial walls relax and the ventricular walls begin to contract.

  • Lasts about 0.05 seconds.

  • All four valves are closed at this time, and the volume of blood in the ventricles does not change.

  • Isovolumetric contraction occurs as the ventricular walls contract, building pressure without volume change.

4. Ventricular Systole

  • Lasts approximately 0.3 seconds.

  • As pressure rises inside the ventricles, it forces the opening of the semilunar valves (pulmonary and aortic).

  • The pressure in the left ventricle must exceed 80 mmHg to open the aortic semilunar valve.

  • The pressure in the right ventricle must exceed 20 mmHg to open the pulmonary semilunar valve.

5. Ventricular Ejection

  • Occurs after the semilunar valves open and lasts about 0.25 seconds.

  • The right ventricle ejects about 70 milliliters of blood into the pulmonary trunk.

  • The left ventricle also ejects about 70 milliliters of blood into the aorta.

  • This volume ejected per heartbeat is called stroke volume.

  • At the end of ventricular ejection, 16 milliliters of blood remains in each ventricle, termed end-systolic volume.

6. Isovolumetric Relaxation

  • Following ventricular ejection, the ventricles relax, and pressure drops quickly.

  • Blood in the pulmonary trunk and aorta begins backflowing toward ventricles, causing the closure of the semilunar valves.

  • At this stage, all four heart valves are closed.

  • The ventricles are relaxed, but the volume of blood inside does not change during this phase.

  • Eventually, the pressure in the atria exceeds that in the ventricles, prompting the opening of the AV valves and initiating a new cycle with ventricular filling again.

Regulation of Blood Flow

  • The opening and closing of the AV valves and semilunar valves regulate blood flow into and out of the ventricles.

    • Ventricular filling occurs when AV valves are open.

    • Atrial systole also allows blood to flow into ventricles when AV valves are open.

    • Ventricular ejection occurs when semilunar valves are open.

    • During isovolumetric contractions and relaxations, all valves are closed, preventing flow and maintaining volume.

Atrial Blood Reception

  • The atria constantly receive blood through veins:

    • Right atrium receives blood via the superior vena cava, inferior vena cava, and coronary sinus.

    • Left atrium receives blood via the four pulmonary veins.

  • There are no valves at the openings of these veins into the atria, allowing for continuous blood flow into the atria during all phases of the cardiac cycle.