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Cardiac Cycle Phases

Shashank Nakate Apr 16, 2019
This write-up deals with information pertaining to the different cardiac cycle phases one-by-one. Functioning of the heart can be understood with the knowledge of actions which take place in these phases.
The cardiac cycle is a series of events in which the heart beats in order to carry out the main functions of receiving and pumping out blood. We are now going to learn more about how the heart pumps blood to our entire body. The cardiac cycle is divided into two phases, i.e., the diastole and systole.
All the events which occur during the cardiac cycle are associated/related to the blood pressure/flow of blood which is initiated by heartbeats. The heart rate is derived from the frequency at which the cardiac cycle occurs. Let us understand more about these phases here.

Phases of Cardiac Cycle

As stated earlier, the length of a cardiac cycle is divided into the diastole and systole phases. The phase in which the heart muscles contract is called the systole, and the phase in which the muscles relax after the systole is known as the diastole. The diastole and systole too are sub-divided into first and second phases. They are explained here:

First Diastole Phase

In the first diastole phase, the heart receives deoxygenated blood from the superior and inferior venae cavae. For blood to enter the heart through the superior and inferior venae cavae, the ventricles and atria relax. With the relaxation of the atria and ventricles, the atrioventricular valve opens. This facilitates the flow of blood to the ventricles.
All the given events are followed by contraction of the SA node*, which in turn contracts the atria. Blood is then transferred from the right atrium to the right ventricle. A valve known as the tricuspid valve is present between the right ventricle and right atrium. It makes sure that the blood flows unidirectionally, and there is no backflow.

First Systole Phase

In the first systole phase, the right ventricle of the heart contracts as a result of impulses received from the 'Purkinje fibers'*. It is followed by the closure of the atrioventricular valves and opening of the semilunar valves.
These actions cause the de-oxygenated blood to get pumped into the pulmonary artery, which carries it to the lungs. The oxygenated blood is then returned to the heart via the pulmonary veins.

Second Diastole Phase

The beginning of the second diastole phase is marked by the closure of the semilunar valves and opening of the atrioventricular valves. The oxgenated blood brought by the pulmonary veins gets accumulated in the left atrium. Simultaneously, the blood present in the vena cava also gets transferred to the right atrium.
This is followed by contraction of the SA node*, which in turn causes contraction of the atria. All these actions cause the left atrium depositing the blood into the left ventricle. The mitral valve prevents the backflow of the blood.

Second Systole Phase

Closing of the semilunar valves and opening of the semilunar valves takes place at the beginning of the second systole phase. Contraction of the left ventricle takes place due to the Purkinje fibers*. This is followed by pumping of oxygenated blood into the aorta.
The blood cannot flow back to the left ventricle from the aorta, since the aortic valve prevents this from happening. The purified/oxygenated blood is distributed to various parts of the body through the aorta.
Impure/deoxygenated blood is brought in by the network of veins to the heart by means of the venae cavae. Knowledge of the cardiac muscle function should help understand the working of heart in a better manner.
*Purkinje Fibers: These are the special group of cells which synchronizes the heartbeat after getting impulses from the SA Node.

*SA Node: Sinoatrial node is the bunch of nerve cells which acts as a natural pacemaker. It is present in the right atrium of the heart. It is because this node that the heart maintains a regular rhythm.