Cardiac Cycle

The cardiac cycle is a continuous sequence of contractions and relaxations of different heart parts in one heartbeat. It lasts for around 0.8 s per cycle - isn't it fascinating to think about how the heart could contract and relax quickly?

Important concepts of the cardiac cycle

Let us approach this topic of the cardiac cycle with a twist, shall we?

First, imagine yourself as a hypothetical microorganism doing a tour of the heart, like you would do your museum tours. However, you need to rely on the blood to help transport you around.

How does blood move from one chamber of the heart to another?

The pressure difference in the heart is responsible for moving blood from one chamber to another or transporting you around the heart. Blood moves from a region of high pressure to low pressure. Contraction of the muscles of one chamber of the heart leads to high pressure, whereas the relaxation of the muscles of another chamber causes low pressure. To explain in simpler terms, you may think of contraction as ‘squeezing’ and relaxation as ‘remaining unchanged’.

Similarly, you’d need to squeeze a bottle to eject the liquid out into the air. By squeezing, you are applying pressure. Hopefully, this serves as another useful analogy to the direction of blood flow in the heart.

We shall wrap up the concept of the direction of blood flow in the heart with a diagram below showing the direction of blood flow from a high-pressure chamber to a low-pressure chamber.

Fig. 1 - The direction of blood flow in the heart

In cardiovascular biology, the contraction and relaxation of heart muscles are called systole (contraction) and diastole (relaxation). We will come across these terms later when we learn about the cardiac cycle.

While you are on your tour of the heart, you may also notice that you travel across the chambers in a specific direction each time you pass through the heart, with the help of door-like structures along the way. These structures are called valves, whose function is to allow blood flow in one direction. You cannot imagine what would happen if blood flowed in all directions - that would not end well.

The valves differ between the heart regions and between the heart and the blood vessels. Refer to our article on the heart for more information. The valves function by opening and closing. Valves open to allow blood to flow and close to stop blood from going the opposite direction. Below is a simplified diagram showing the role of valves in controlling the direction of blood flow.

What causes valves to open or close? Pressure differences influence the opening and closing of valves. Valves open when the pressure difference follows the direction of blood flow and close when the pressure difference is against the direction of blood flow.

What happens during the cardiac cycle?

A single cycle of cardiac activity can be divided into two phases - systole and diastole. The cardiac cycle is then further divided into three stages - atrial systole, ventricular systole, and ventricular diastole. Below is a diagram that shows the big picture of the cardiac cycle events.

Fig. 2 - The cardiac cycle

To help visualise the cardiac cycle better, below is an animation of the heart pumping as it undergoes a series of cardiac cycles.

Figure 3

The main exhibits of your heart tour are the valves (atrioventricular and semilunar), chambers (atria and ventricles) and blood. On one cardiac cycle, which is one tour around the heart, we will see how these exhibits come into play with each other across the three stages.

Pressure changes in the left side of the heart during one cardiac cycle

Pressure changes influence the events of the cardiac cycle. Since pressure can be quantified, we can plot graphs of pressure changes at the different regions of the heart against time during one cardiac cycle.

Fig. 4 - Cardiac cycle and the “Lub” and “Dub” sound

Pressure changes in the left ventricles are greater than in the left atria due to the thicker muscular walls. As the aorta carries blood at high pressure, the pressure only changes when blood flows through it during ventricular systole.

How does cardiac output relate to the cardiac cycle?

Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute. This is how we quantify ventricular systole. The unit for cardiac output is in ${\mathrm{cm}}^3{\min }^{-1}$, and cardiac output depends upon two factors:

• The heart rate is the rate at which the heart beats, which is the number of heartbeats per minute. So, the larger the value, the faster the heart beats

• The stroke volume is the volume of blood pumped out at each beat. So, the larger the values, the faster the heart beats

Given how the heart rate and stroke volume affect cardiac output, we can construct an equation to derive cardiac output from heart rate and stroke volume as:

$\mathrm{Cardiac}\mathrm{output}=\mathrm{heart}\mathrm{rate}\mathrm{x}\mathrm{stroke}\mathrm{volume}$

Based on the stated equation, the higher the heart rate and stroke volume, the higher the cardiac output. In other words, a heart that pumps fast and pumps out high volumes of blood yields high cardiac output.