Online College Courses for Credit

Cardiac Cycle Overview

Cardiac Cycle Overview


This lesson will explain the steps that occur when your heart muscle contracts. It will investigate the cardiac cycle and the cardiac conduction system.

See More
Fast, Free College Credit

Developing Effective Teams

Let's Ride
*No strings attached. This college course is 100% free and is worth 1 semester credit.

29 Sophia partners guarantee credit transfer.

312 Institutions have accepted or given pre-approval for credit transfer.

* The American Council on Education's College Credit Recommendation Service (ACE Credit®) has evaluated and recommended college credit for 27 of Sophia’s online courses. Many different colleges and universities consider ACE CREDIT recommendations in determining the applicability to their course and degree programs.


The Cardiac Cycle

Source: Heart Image from Adobe Illustrator; Public Domain

Video Transcription

Download PDF

Welcome to this lesson today on the cardiac cycle overview.

Today we will be discussing the processes that occur in the cardiac cycle.

The cardiac cycle is a sequence of events that occurs when the chambers of our heart contract and relax. We have four chambers in our heart. We have our right atrium, our left atrium, our right ventricle, and our left ventricle.

When these chambers contract and relax, it acts to help pump blood throughout our body, and the contraction and relaxation-- I can't speak today-- contraction and relaxation occur at the same time on each side of the heart.

For example, when the right atrium relaxes, the left atrium relaxes. When the right atrium contracts, the left atrium contracts.

The two phases of the cardiac cycle, then, are the contraction phase and the relaxation phase.

The contraction phase is called systole, and the relaxation phase is called diastole.

We will take a look at what happens in this cardiac cycle, this sequence of relaxation and contraction of the different chambers of the heart.

Let's start with our atria. As I mentioned, we have a right and left atrium. When our atria contract, it's going to push blood into the ventricles. We have something that separates our atrium from our ventricles, and it's called an atrial ventricular valve. Here's our atrial ventricular valves, here and here

So our atrial ventricular valves will open, the atria will contract on both the right and left side at the same time, and that will push blood into the ventricles. From there, the ventricles are then going to contract and, depending on which side of the heart it is, it's either going to pump blood into the pulmonary artery or the aorta.

On this side of our heart, this is going to be our left ventricle. From here, it's going to pump blood up through our aorta. So when the ventricle contracts, it's going to contract and push blood up into the aorta.

This is our right ventricle. And what is going to happen here is that this ventricle will contract, and it's going to push the blood up through this pulmonary valve, and then the blood will make its way to the lungs to collect more oxygen. Then the ventricles will relax, and then the atria will begin to fill again for another cycle.

So basically, we have blood moving from our atria to our ventricles, and then the process will happen again.

Basically, the valves that I mentioned, the atrial ventricular valves and then also the valves that we have-- our aortic valve and our pulmonary valves-- they act to prevent back flow.

If you listen to your heart, it makes this lub-dub sound. And this lub-dub sound is the sound that your heart makes because of the closing of the heart's valves. That first part-- the lub sound-- is made by your AV valves closing simultaneously. And the dub sound is caused by the closing of the aortic and pulmonary valves simultaneously.

This lesson has been a brief overview on the cardiac cycle.

Terms to Know
Cardiac Cycle

All of the events that occur during one heart beat that drive blood flow into and out of the heart; a series of well-timed pressure and volume changes within the chambers of the heart that drive blood flow throughout its chambers.


A term used to describe a decrease in pressure and work; measured in millimeters of mercury (mmHg).


A term used to describe an increase in pressure and work; measured in millimeters of mercury (mmHg).