The Cardiac Cycle: Phases, Explanation & Terms

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Heartbeat and Heart Contraction Coordination

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:07 Cardiac Cycle
  • 1:44 Diastole
  • 3:26 Systole
  • 5:17 Lesson Summary
Save Save Save

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Speed Speed

Recommended Lessons and Courses for You

Lesson Transcript
Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

In this lesson, you will learn what happens during each heartbeat, or cardiac cycle. You will also learn that the heartbeat involves periods of relaxation and contraction, known as diastole and systole, respectively.

Cardiac Cycle

'Lub-dub,' 'lub-dub' - This is the familiar sound of the heartbeat. You can think about the things that make your heart race, like watching a scary movie or catching a glimpse of your true love, but what really happens during a heartbeat and what is going on inside the heart while it's beating away? These are the topics we will tackle in this lesson.

We know that the human heart has four chambers - two atria on top and two ventricles below. We also know that the heart is equipped with one-way valves separating the chambers and the major arteries. These valves prevent backflow of blood. When we look at the flow of blood through the heart, we see that it first enters the atria and then passively flows into the ventricles. When the ventricles are almost full, the atria contract and they push as much blood as possible into the ventricles. Now, the ventricles are almost full, and they're under very high pressure. At this time, they contract and push blood out of the heart and into the arteries.

Diagram showing the flow of blood in the heart
Heart Blood Flow

What I just described is one cardiac cycle, or the sequence of events in a single heartbeat. Since the average heart beats about 75 beats per minute, the length of each cardiac cycle is less than one second. But, during that brief period of time, there's a lot of pressure changes taking place in the heart. Let's look at how pressure rises and falls in the heart as it moves through the different phases of the cardiac cycle.


In a healthy heart, the left and right ventricles contract and relax simultaneously. Diastole is the term used to describe the relaxation of the heart. One thing to point out here is that even though we know that the atria also contract and relax during the cardiac cycle, the ventricles are the primary pumps of the heart; therefore, the term diastole typically refers to the relaxation of the ventricles.

When the heart is in this state of relaxation, or diastole, the pressure within the heart is low. This is when blood is passively flowing through the atria and into the ventricles, almost like water pouring out of a spigot and filling a water balloon. We remember that the atrioventricular valves, or the AV valves, are open during this diastolic phase to allow the blood to move from the atria to the ventricles freely. We also remember that the semilunar valves, which are the valves that guard the arteries leaving the heart, are closed at this time.

What we see as a result is that the ventricles rapidly fill during diastole, and pressure within the ventricles begins to rise, along with the volume. At the end of diastole, the ventricles have filled quite a bit and they're almost full. Then, the atria contract, adding even more volume to the ventricles. The volume of blood in the ventricles at the end of diastole is referred to as the end-diastolic volume. You can imagine that an increase in the end-diastolic volume results in more stretching and pressure within the ventricle, and this results in an increased contraction strength.

During diastole, the atrioventricular valves are open.
Heart During Diastole


At this point, the ventricles are under very high pressure, and now we move into the next phase of the cardiac cycle, called systole. This is a term used to describe the contraction of the heart. When the ventricles contract, the AV valves snap shut and the semilunar valves open wide as blood is pushed out of the ventricles and into the large arteries leaving the heart. It's interesting to note that systole is shorter than diastole. In other words, the contraction of the heart is shorter than the filling time.

To unlock this lesson you must be a Member.
Create your account

Additional Activities

Cardiac Cycle Activity

In this activity, students will assume the role of a doctor with a young cardiac patient. The doctor needs to explain the cardiac cycle to the child in order for the child to have a better understanding of his/her condition.

The goal of this activity is for students to create a learning tool to teach the concepts from the lesson to younger students/children. In order to do this, students will need to understand the material deeply and be able to express the information in simple terms. Teaching a topic is a great way to ensure that the topic is understood.


  • Take notes on the lesson. This could be in bullet form, outline form, or mental map form. Make sure to study your notes so you understand all the information given about the cardiac cycle.
  • Research the cardiac cycle and find at least one more fact that is not given in the lesson to include.
  • Decide on an appropriate method to deliver the information to the child. Remember this method should be creative and engaging to keep the child's attention while the information is being given. You could:
    • Write a song
    • Compose a poem
    • Create a diagram
    • Make a flow chart
  • Whatever method is chosen, make sure to include:
    • An explanation of what the cardiac cycle is
    • Definitions for major terms:
      • Diastole
      • End-diastolic volume
      • Systole
      • End-systole volume
    • At least one additional fact not given in the lesson
    • A summary that quickly sums up the cardiac cycle in a single sentence

If possible, students should record themselves executing the lesson (singing the song, reciting the poem or explaining the diagram, etc). Additionally, if possible, students should present their information to others (friends, family or classmates if available).

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use

Become a member and start learning now.
Become a Member  Back
What teachers are saying about
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account