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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.
Study tip: Students may get confused between opening and closing vs contraction and relaxation when it comes to the cardiac cycle. Remember, the heart valves and muscles are the two key players. Valves are like doors - they open and close. On the other hand, muscles of the heart chambers are squishy - they contract and relax.
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.
On your tour of the heart, you may notice the hypnotic ‘lub-dub’ sound in the background throughout the time. Where does this sound come from? This ‘lub-dub’ sound comes from the closing of valves. That sound you would hear whenever you shut a door? As two surfaces hit each other? Likewise for valves. ‘Lub’ is the sound when the atrioventricular valves shut, whereas ‘dub’ is when the semilunar valves shut.
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.
Study tip: Remember which curve belongs to which part of the heart, as exams may test your understanding. Understanding the rationale behind the pressure changes to help you in your exam.
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 , 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:
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.
A woman whose heart completes one cardiac cycle in 0.5 s has her stroke volume measured as 70 during that timeframe. Calculate her cardiac output.
We answer this question by first determining her heart rate and stroke volume.
Study tip: Make sure to convert the heart rate units into min-1 before you calculate the cardiac output value.
After calculating the heart rate and stroke volume, we substitute these values into the cardiac output equation.
Cardiac Cycle - Key takeaways
- Blood moves from a chamber of higher pressure to a chamber of lower pressure during the cardiac cycle. Valves open and close to make sure blood flows in a regular direction; the opening and closing of valves also depend on the pressure difference across the different chambers of the heart.
- There are three stages to the cardiac cycle: atrial systole, ventricular systole and ventricular diastole. Atrial systole is when blood is pumped into the ventricles, whereas ventricles pump blood into the aorta that transports blood to the rest of the body during ventricular systole. Both the atria and ventricles relax during ventricular diastole, as blood fills the heart at this stage.
- Heart sounds, the characteristic ‘lub-dub’, come from valves closing. Closing the atrioventricular valves generates the ‘lub’ sound, whereas ‘dub’ sound comes from the semilunar valves closing.
- As pressure changes influence the cardiac cycle, they can be quantified and plotted into graphs then compared between the heart regions.
- Cardiac output is a measure of ventricular systole. The value of cardiac output can be calculated using the formula cardiac output = heart rate x stroke volume.
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Frequently Asked Questions about Cardiac Cycle
What is the cardiac cycle?
A cardiac cycle is a continuous sequence of contractions and relaxations that occur in the heart in one heartbeat.
How to calculate the cardiac cycle length from heart rate?
Heart rate (HR) = number of beats per minute
Cardiac cycle length = length of time of events in one heartbeat
If HR = 60beats/minute = 60 beats/ 60s = 1beat/s,
cardiac cycle length = reciprocal of HR
= 1 s/ 1 beat
= 1s
How long does a cardiac cycle last?
Approximately 0.8s.
Why is the cardiac cycle important?
The cardiac cycle is important for a smooth blood flow to enable efficient transport and exchange of substances with all the cells in the body.
What are the two phases of the cardiac cycle?
Systolic and diastolic phases.
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