The cardiac cycle describes the sequence of events in the heart from one heartbeat to the next. It consists of two main periods - systole and diastole. Systole involves ventricular contraction and blood ejection, while diastole involves ventricular relaxation and filling. The cardiac cycle can be divided into 7 phases: atrial contraction, isovolumetric contraction, rapid ejection, reduced ejection, isovolumetric relaxation, rapid filling, and reduced filling. Each phase is characterized by specific events in the heart such as valve openings/closings and changes in pressure.

1. CARDIAC CYCLE
CARDIAC CYCLE PHASES
CARDIAC CYCLE:
The cardiac cycle is the performance of the human heart from the ending of one heart beat to the
beginning of the next. One cardiac cycle is complete in 0.8 sec.
In cycle atria is contract for 1sec and relax for 7sec and ventricle contract for 3 sec and relax for
5sec. One cardiac cycle is completed when the heart chamber fill with blood and blood is pump
out from the heart.
It consists of two period systole and diastole.
Systole: The heart muscles relax and refills with blood called systole.
Diastole: A period of robust contraction and pumping of blood called diastole.
PHASES OF CARDIAC CYCLE
The cardiac cycle is usually divided into seven phases.
Phase 1: Atrial Contraction
Phase 2: Isovolumetric Contraction
Phase 3: Rapid Ejection
Phase 4: Reduce Ejection
Phase 5: Isovolumetric Relaxation
Phase 6: Rapid Filling
Phase 7: Reduce Filling
ATRIAL CONTRACTION ( PHASE 1 )
 A-V VALVES OPEN
 SEMILUNAR VALVES CLOSED
 This is the first phase of cardiac cycle.
 Represent electrical depolarization of the atria.

2.  Atrial depolarization initiates contraction of the atria musculature.
 As the atria contract, the pressure within the atrial chambers increase, which forces more
blood flow across the open atrioventricular AV valves, leading to a rapid flow of blood
into the ventricle.
 AFTER ATRIAL CONTRACTION is completed, the atrial pressure begins to all fall
causing a pressure gradient reversal across the valves.
 This cause the valve to float upward before closure.
ISOVOLUMETRIC VENTRICULAR CONTRACTION
PHASE ( PHASE 2 )
During this phase, electrical activation of the ventricles begin to contract and the following event
occurs;
 Ventricular pressure rise and it becomes greater than the arterial pressure.
 AV Valves are closed it is because of the pressure difference.
 Semilunar and aortic valve do not open so there is no emptying and no volume change.
 The closure of valve result in first heart sound.
 On ECG is represented by QRS complex.
RAPID EJECTION ( 3RD
PHASE )
 The Semilunar valve open at the beginning of this phase.
 But Atrioventricular valve remain closed.
 This phase represents initial , rapid ejection of blood into the aorta and pulmonary artery
from the left and right ventricles respectively.
 When the intraventricular pressure exceed the pressure within the pulmonary artery and
aorta, which cause the aortic and pulmonary valve to open.
 Ventricular continue contracting pressure increase greater than aortic pressure 80mmhg.
 Opening of aortic valve .Most of stroke volume (almost 70ml) ejected during this phase
and reduce left ventricle volume.
REDUCE EJECTION ( 4TH
PHASE )
 In this phase ventricular volume is further decrease.
 Ventricular pressure is slightly decrease due to ejection of most blood in preceding phase.
 Aortic pressure is slightly decrease.

3.  Semilunar valves are open and the atrioventricular valve are closed.
 There are no sound in this phase.
 Arial pressure is still increasing due to venous return.
In ECG show most of the ascending limb and the top of the T wave.
ISOVOLUMETRIC RELAXATION PHASE ( 5TH
PHASE)
 At the end of T wave which shows that replorization of the ventricle is complete.
 Pressure drop which result in aortic valve closure, followed by the closure of pulmonary
valve.
 Second heart sound produced in this phase.
 During most of the Isovolumetric ventricular relaxation phase AV valves remain closed.
 Ventricles are now relaxed and because of this ventricular pressure decrease.
 As all the valve are closed in this phase so ventricular volume is constant that is why this
phase is called Isovolumetric phase.
RAPID VENTRICULAR FEELING (6TH
PHASE)
 Ventricular pressure less than atrial pressure and the mitral valve open.
 With the opening of mitral valve, ventricular feeling from the atrium begins.
 As blood continuously flow into smaller arteries aortic pressure continues decrease.
 As blood flow rapidly from the atria into the ventricle it causes third heart sound which is
normal in children but in adult it is associated with disease.
REDUCE VENNTRICULAR FILLING ( 7TH
PHASE)
 It is the longest phase of cardiac cycle.
 During this phase, ventricular filling continues but it is at a slower rate.
 This phase is called diastasis.
 The time require for diastasis and ventricular filling depends upon heart rate.
 For example increase in heart rate cause decrease time available for ventricular refilling.
 Decrease end diastolic volume and decrease stroke volume.