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The Cardiac Cycle: Phases, Explanation & Terms

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  • 0:07 Cardiac Cycle
  • 1:44 Diastole
  • 3:26 Systole
  • 5:17 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

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

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.

Diastole

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

Systole

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.

So, when you are resting on the couch watching a movie, you could say that most of the time your heart is passively filling with blood and in diastole. However, if the movie becomes scary and shows a zombie attack and your heart starts to race, when this happens, the diastolic phase is the one that shortens during the cardiac cycle, so your heart spends less time relaxing and more time contracting.

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