Cardiac cycle explained: cardiac cycle phases, ECG, graph

The heart is a pumping machine which collects impure blood from our body and pumps it to the lungs for purification. The pureblood is again collected in the heart which once again is pumped to different parts, muscles, cells and tissues of our body. All these series of events happen smoothly in a cycle and are known as the cardiac cycle. In this article, we will try to learn what is the cardiac cycle and what those curves and plateaus denote in a cardiac cycle.

Cardiac cycle explained

Our heart consists of four chambers, two of them lie on the left side and the other two lies on the right side. We call it as Right atrium, Right ventricle Left atrium, or Left ventricle (see figure above). Each of the chambers has an opening to allow blood to flow inside and each opening had a lid that checks the flow of blood. We call this lid a valve. Our heart has four-valve.

1. Between the left atrium and left ventricle is the mitral valve.
2. On the right side between the right atrium and right ventricle is the tricuspid valve.
3. The right ventricle opens in the pulmonary artery and in between them there is a pulmonary valve.
4. Similarly, the left ventricle opens in the arch of the aorta and the aortic valve lies between them.

Cardiac cycle phases

The heart is the most hardworking organ inside our body which works to supply blood to each and every corner of our body through the circulatory system. We can electronically measure this cardiac cycle and in science, we call this process measuring ECG (Electro Cardio Gram). Broadly, the cardiac cycle passes through two phases:
Diastole: it is a period of relaxation of the heart chamber when refilling of the heart occurs.
Systole: The period of contraction of the heart chamber and blood is pumped out of the heart.

These two phases are subdivided into seven phases. These are:

1. Atrial systole.
2. Isovolumetric contraction.
3. Rapid ventricular ejection.
4. Reduced ventricular ejection.
5. Isovolumetric relaxation.
6. Rapid ventricular filling.
7. Reduced ventricular filling.

Don’t get confused by all these terms as we will discuss them in detail and I will try to make it as simple as possible. At the end of this subheading, you will find a table of all phases and their events. So, let us start one by one.

Also read: Which organ is called second heart of the body & why?

Phase#1 Atrial systole

Atrial systole is the phase of atrial contraction. During this phase, the atrium contracts to pump blood in the ventricles. But only 20% of ventricle filling occurs by atrial systole, the rest of the 80% of ventricular filling has been done passively even before the onset of atrial contraction. This active filling of ventricles becomes valuable during physical activity.

When the pressure in the atrium increases, blood rush into the ventricles through the opened mitral valve. During left atrium contraction, pressure and volume are transferred into the left ventricle through opened mitral valve. Remember aortic valve is closed because the pressure in the aorta is greater than the pressure in left ventricle at this moment.

To understand it clearly let us once again go back to the above figure and observe the arrow. As you can see the arrow shows the direction of flow of blood from the superior and inferior Vena cava that supplies blood into the right Atrium and the arrow on the pulmonary Veins shows the direction of flow of blood to the left Atrium.

As both the atrioventricular valves are open blood from left and right Atrium goes into the left and right ventricles.

Phase#2 Isovolumetric contraction

It is the initial phase of the ventricle systole, it means ventricles are in the state of contraction. So, in this phase, the contraction ventricle starts and the pressure inside the chamber starts to build.

But initially, the pressure inside the ventricle is not sufficient enough to push open the semilunar valve. Soon this initial pressure rises above the pressure of the atrium and this results in the closure of the atrioventricular.

Phase#3 Rapid ventricular ejection

This is the second phase of the ventricular systole and slowly pressure inside the ventricles increases. As the pressure builds up soon it reaches a point where it pushes open the arctic and pulmonary valves and pumps out blood from the heart.

Since the blood is ejaculated rapidly from the ventricle this phase is called as Rapid ventricular ejection phase. During this phase, 70% emptying of blood occurs.

Phase#4 Reduced ventricular ejection

Contrary to the above it is a phase of reduced ventricular ejection. It is the last phase of the ventricle systole during which the pressure inside the ventricles decreases and the rest 30% of the blood is emptied.

Phase#5 Isovolumetric relaxation phase

A sub-phase of ventricle diastole during which the ventricle comes to a state of relaxation and the isovolumetric relaxation phase is the early phase of ventricle diastole.

As the muscles of the ventricle start to relax the pressure inside the ventricle dips. When it decreases below the pressure of the pulmonary trunk and aorta, the blood rushes back towards the heart. The backward flow of blood causes closure of the semilunar valve.

During this phase, the atrioventricular valves remain closed and there is no change in the volume of the blood in the ventricle and this is why it is called an isovolumetric relaxation phase.

Phase#6 Rapid ventricular filling phase

Rapid ventricular filling phase is the second phase of ventricle diastole when muscles of the ventricle further relax and pressure decreases further. Eventually, the pressure inside ventricle drops to a point where it becomes lower than the pressure inside the atria.

As a result blood from the atrium push opens the atrioventricular valve and blood rushes into the ventricles. This happens rapidly so it is called a Rapid ventricle filling phase.

Phase#7 Reduced ventricular filling

It is the longest phase of the cardiac cycle during which the filling of ventricle continues slowly.

Table of phases of the cardiac cycle

Cardiac cycle ECG

We can study these phases of the cardiac cycle by analysing the electrical activity of the heart. This process has been termed an electrocardiogram (ECG/ EKG). I have already covered this topic extensively in one of my articles How to read ECG: Electrocardiogram Simplified.

Here’s a summary of what an ECG wave contains.
P stands for atrial systole.
Q, R, S complex stands for ventricle systole.
T, U stand for diastole.

Keep Reading: Difference Between Stroke and Heart Attack

Sunit. S. Ekka

Dr Sunit Sanjay Ekka is a physiotherapist in practice for the last 15 years. He has done his BPT from one of the premium Central Government physiotherapy colleges, ie, SVNIRTAR. The patient is his best teacher and whatever he gets to learn he loves to share it on his Youtube channel and blog.