“The Cardiac cycle includes all physiological events associated with the single heartbeat including electrical events, mechanical events( pressure and volume) and heart sounds.”In the cardiac cycle the period of time begins with contraction of atria and ends with ventricular relaxation.
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What Is Cardiac Cycle
The cardiac cycle is defined as the sequence of events of alternative contraction and relaxation of atria and ventricle in order to pump blood to all over the body. It intiates at begining of one heart beat and ends at the begining of another heart beat. The cardiac cycle includes 1)Diastolic phase (Relaxation)- the period of relaxation that occurs when chambers fill with blood and 2) Systolic Phase (Contraction)-the period of contraction that the heart undergoes while it pumps blood into circulation is called systole.
A healthy heart of a human beats 72 times per minute it means that there are 72 cardiac cyle in a minute. The cardiac cycle includes complete and contraction and relaxation of the atria and ventricles and the cycle lasts about 0.8 second.
Four stages of the cardiac cycle as follow:
Atrial Systole: About 0.1 second
Ventricular Systole: About 0.3 second
Atrial Diastole: About 0.7 second
Ventricular Daistole: About 0.5 second
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Phases Of Cardiac Cycle
Basic Principles:
Blood flows from higher to lower pressure.
Contraction increases the pressure
Valves open/close according to pressure gradient.
- Atriole Diastole: Atriole diastole is the first phase of the cardiac cycle . During atriole diastole blood enters into the right atrium through the superior vena cava and blood enters into the left atrium through pulmonary veins. In the early part of the phase the atrioventricular valves are closed and blood stores in the atrium.
- Atriole Systole:In this phase sinuatrial node (SAN) intiates the impulse. This electric depolarization results in simultaneous contraction of atria. Contraction of atria put the pressure on blood and forces it to go down to lower chamber (ventricles) of the heart. In this phase artrioventricular valves are open and blood goes through this valve. At the start of the atriole systole, the ventricles are normally filled with approximately 70-80 % of the blood.
- Ventricular Diastole:At the early stage of this phase both the atrioventricular and semilunar valves are close. During this phase there is no change in amount of blood in the ventricle. This phase is known as isovolumetric relaxation.Eventually the ventricular pressure decreases. It becomes less than atrial pressure and atrioventricular valves open. Diastole of ventricles results in filling of ventricle with blood which is often referred to as the rapid filling of ventricles. This represents most of the blood in the ventricles before they contract. A small amount of blood flows directly into the ventricles from the vena cava.Towards the end of the ventricular dilation any remaining blood in the atria is pumped into the ventricles. The total amount of blood presents in the ventricle at end-diastoleis called the end-diastolic or preload volume.
- Ventricular Systole:
As the ventricle contracts the pressure increases more than the coreesponding atrium. It results in the closure of the atrioventricular valves. In this phase ventricles are in a state of isovolumetric contraction. As the ventricular pressure increases, the semilunar valve opens and allow blood to flow out of the ventricle. This is the ejection phase of the cardiac cycle.
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