Cardiac Cycle

Cardiac Cycle

The cardiac cycle is the process of your heart beating. It involves different parts of your heart contracting and relaxing all in one heartbeat. The entire process lasts for about 0.8 seconds per cycle. It's pretty amazing to think about how your heart can work so quickly! This process is sometimes called the heartbeat cycle, and it's a crucial part of your body's cardiovascular system. In fact, understanding the cardiac cycle is important for keeping your heart healthy. So, next time you feel your heart beating, take a moment to appreciate the incredible process that's happening inside your body!

Important concepts of the cardiac cycle

Let's take a unique approach to learning about the cardiac cycle. Imagine you're a tiny microorganism taking a tour of the heart, relying on blood to transport you around. Pretty cool, right?

So, how does blood move from one chamber of the heart to another? Well, it's all about pressure. Blood flows from areas of high pressure to low pressure. When one chamber of the heart contracts, it creates high pressure, while another chamber relaxes, causing low pressure. Think of the contraction as squeezing and the relaxation as remaining unchanged.

To make it simpler, let's use a bottle as an analogy. When you squeeze a bottle, you apply pressure and eject the liquid out into the air. It's the same concept with blood flow in the heart.

To sum it up, blood moves from high-pressure chambers to low-pressure chambers, and this helps the microorganisms like you get around the heart. Check out the diagram below to see the direction of blood flow between chambers.

The direction of blood flow in the heart

As we continue our tour of the heart, it's important to understand the terms systole and diastole. These are the medical terms for the contraction and relaxation of heart muscles. You'll come across these terms again when we talk about the cardiac cycle.

As you move through the heart, you'll notice that you're traveling in a specific direction each time, thanks to door-like structures called valves. These valves make sure that blood flows in one direction only. If blood flowed in all directions, it would be a disaster!

There are different types of valves in the heart and blood vessels, which you can learn more about in our article on the heart. Valves open to allow blood to flow and close to stop blood from going in the wrong direction. Check out the diagram below to see how valves control the direction of blood flow.

It's important to remember that valves open and close based on pressure differences. When the pressure difference follows the direction of blood flow, valves open. When the pressure difference is against the direction of blood flow, valves close.

So, to sum it up, valves are like doors that open and close to control the direction of blood flow, while the muscles of the heart chambers contract and relax during systole and diastole. Remembering these key players and their functions will help you understand the cardiac cycle better.

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.


The cardiac cycle
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.

The heart pumping blood

Now, let's explore the cardiac cycle, which is like a tour around the heart. During one cardiac cycle, we'll see how the valves, chambers, and blood work together across three stages.

Stage 1: Atrial Systole

During the first stage, the atria (upper chambers) contract, and the ventricles (lower chambers) relax. This causes the atrioventricular valves to open, allowing blood to flow from the atria to the ventricles. The semilunar valves remain closed during this stage.

Stage 2: Ventricular Systole

In the second stage, the ventricles contract, and the atria relax. This causes the atrioventricular valves to close, producing the 'lub' sound. The semilunar valves then open, allowing blood to be pumped out of the heart and into the arteries.

Stage 3: Diastole

During the third stage, all chambers relax, and the semilunar valves close, producing the 'dub' sound. This relaxation allows the heart to refill with blood, and the cycle begins again.

Throughout the cardiac cycle, the valves, chambers, and blood work together to keep blood flowing in the right direction. And that 'lub-dub' sound you hear? It's just the sound of the valves closing, like doors shutting in your heart museum tour.

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.

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?

To calculate the cardiac output of the woman in question, we first need to determine her heart rate and stroke volume.

Heart rate = 60 s/min ÷ 0.5 s/cardiac cycle = 120 beats/min

Stroke volume = 70 ml

Now we can substitute these values into the cardiac output equation:

Cardiac output = heart rate x stroke volume
Cardiac output = 120 beats/min x 70 ml/beat
Cardiac output = 8,400 ml/min or 8.4 L/min

Therefore, the woman's cardiac output is 8.4 L/min.

In summary, understanding the cardiac cycle is important in understanding how the heart functions. Blood flow, valves, pressure differences, and heart rate all play a role in the cardiac cycle. Cardiac output is a measure of ventricular systole, which can be calculated using the equation cardiac output = heart rate x stroke volume.

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 heartbeatIf 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.

Join Shiken For FREE

Gumbo Study Buddy

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime