Cardiac Cycle

Definition

The mechanical events in the heart that occur from the beginning of one heart beat to the beginning of the next are called the cardiac cycle.

Electrical events of the cardiac cycle

Electrical events of the heart is immediately followed by mechanical event of the heart. The events that occurs during one cardiac cycle include filling up of heart with blood and pumping out of blood from heart. At rest the process takes approximately 0.883 seconds ( Heart rate = 75 bpm)

During each cardiac cycle, there will be changes in,

                                        - pressure

                                        - volume

                                        - valves closure / opening

                                        - blood flow

Mechanical events of the cardiac cycle

There are two phases in the cardiac cycle,

                    1. Relaxation phase

                    2. Contraction phase

The contraction phase is called systole ( 0.3 secs)

The relaxation phase is called diastole ( 0.5 secs)

Divisions of the cardiac cycle are,

                    - systole

                    - diastole

The phases of the cycle are identical in both halves of the heart. Atria and ventricle go through separate cycles of the systole and diastole. When the term systole and diastole alone use, it usually refers to ventricles, unless specified for atria.

Phases of the cardiac cycle

1. Atria ( both ) fill blood during atrial diastole

    - blood pressure in atria increase causing AV valve to open

    - blood flows into ventricles passively ( no contraction needed initially)

2. Atria contract forcing last drop of blood into ventricles ( atrial systole )

        Approximately 70% of ventricular filling occurs passively and rest of the 30% due to atrial contraction.

3. At this point the ventricles are relaxed allowing blood flow to flow in ( ventricular diastole )

4. Next the ventricles contracts causing the semilunar valves to open, forcing the blood out of the heart. ( left ventricle - aorta to the body/ right ventricle - pulmonary artery to the lungs) ( ventricular systole )

• semilunar valves then close preventing blood flowing back into the ventricles
• the cycle begins again ( with atrial diastole - blood flowing into the atria )

Mechanical events of the cardiac cycle produces sequential changes in pressures resulting blood flow in the heart chambers and blood vessels enabling the heart to pump blood to peripheral tissues.

There are 5 phases of cardiac cycle in a healthy adult.

1. atrial systole 
2. isovolumetric ventricular contraction
3. ventricular ejection
4. isovolumetric  ventricular relaxation
5. ventricular filling

• During diastole - passive filling of the relaxed left ventricle

                During atrial contraction a small amount of blood is pushed into the ventricle. 

• During isovolumetric contraction - the mitral valve closes. The ventricular pressure rises toward aortic pressure
• Ejection phase starts as the aortic valve opens. The ventricular contents are ejected during this time.
• Isovolumetric relaxation begins as the aortic valve closes ventricular pressure falls.

Events during late diastole ( ventricular filling )

• Mitral and tricuspid valves between the atria and ventricles ( AV valves ) open and aortic and pulmonary valves are closed.
• Blood flows into the heart throughout diastole, filling the atria and ventricles
• The rate of filling declines as the ventricles become distended as the cusps of the AV valves drift toward the closed position.
• The pressure in the ventricles remains low. About 70% of the ventricular filling occurs passively during diastole.

Events during atrial systole

• Contraction of the atria, pump additional amount of blood into the ventricles.
• Contraction of the atria muscle narrows the opening of the superior and inferior vena cava and pulmonary veins, and the intertia of the blood, moving toward the heart tends to keep blood in it. This minimizes the regurgitation of blood back into the veins.
• Atrial contraction, the second period of diastole during which there is significant blood flow is when the ventricles are actively filled by blood from atrial contraction. 
• Atrial contraction has a "pump - priming " action by increasing the ventricular pressure immediately prior to systole.
• This increases the strength of ventricular contraction.

Events during ventricular systole ( isovolumetric contraction )

• At the start of ventricular systole, the AV valves close.
• The intraventricular pressure rises sharply and markedly as the myocardium squeezes the blood in the ventricle.
• This is isovolumetric ventricular contraction
• It lasts , until the pressure in the left and right ventricles exceed the pressures in the aorta and the aortic and pulmonary valves open.
• During this period, the AV valves bulge into the atria, causing a small but sharp rise in atrial pressure.

Events during ventricular ejection 

• When the aortic and pulmonary valves open, ventricular ejection begins.
• Ejection is rapid at first, slowing down as systole progresses.
• The intraventicular pressure rises to a maximum and then declines somewhat before ventricular systole ends.
• The AV valves are pulled down by the contractions of the ventricular muscle, and atrial pressure drops.
• The amount of blood ejected by each ventricle per stoke at rest is 70 - 90 ml.
• Thus , about 50 ml of blood remains in each ventricle at the end of systole. ( end - systolic ventricular volume )

Events during early diastole ( isovolumetric ventricular relaxation )

• Soon after the ventricular muscle is fully contracted, ventricular pressures drop more rapidly and markedly.
• It ends when the momentum of the ejected blood is overcome and the aortic and pulmonary valves close, setting up transient vibrations in the blood and blood vessels walls.
• After the valves are closed, pressure continues to drop rapidly during the period.
• This is called isovolumetric ventricular relaxation
• It ends when the ventricular pressure falls below the atrial pressure and the AV valves open, permitting the ventricles to fill again.( beginning of late diastole )
• Filling is rapid at first , then slows as the next cardiac contraction approaches.
• Atrial pressure continues to rise after the end of ventricular systole until the AV valves opens, then drops and slowly rises again until the next atrial systole.