Systole: Definition & Measurement

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

This lesson defines what the term systole means and how it occurs in the body. We will take a brief look at the function of the heart in order to fully understand what is happening during systole. After reading the lesson, there is a short quiz to check your knowledge.

Systole Definition

Many medical shows have been on TV over the years. Most of the shows take place in the emergency room of a hospital where they show intense life-and-death situations. You may recall seeing a flat line on the monitor the doctors were reading and hearing the doctors say the word asystole when that happened.

Flat line indicates that the heart is no longer beating
Picture of flat line on electrocardiogram

Asystole means without heart contractions. Systole means heart contractions. I bet it all makes more sense now why the doctors used that term in the show. What they were saying is that the person no longer has a heartbeat, which of course ultimately results in death. Now we'll spend a little more time discussing systole.

What Happens During Systole?

So, we know at this point systole means heart contraction. Let's quickly review some of the structures and functions of the heart to discuss when systole occurs. Remember that the heart is a muscle that is divided into four chambers. The upper chambers are the atria and the lower chambers are the ventricles. The ventricles are considerably larger than the atria.

The chambers of the heart and direction of blood flow
Diagram of heart chambers

The atria contract together during a heartbeat and then relax at the same time. When the atria contract this is referred to as atrial systole. During atrial systole, the blood is forced out of the atria and into the ventricles. The atria will then relax, and now the ventricles will simultaneously contract. The ventricles will also relax at the same time, and this ends one heartbeat. When the ventricles contract this is known as ventricular systole. Ventricular systole forces the blood to leave the ventricles and enter either the pulmonary arteries or aorta. Since the ventricles are much larger and they have to send the blood further, the contraction of the ventricles is much stronger than the contraction of the atria. If the term systole is used alone, then it refers to ventricular systole since it is the strongest contraction. Diastole is the term used for when the heart muscles relax between contractions or heartbeats.

How Systole is Measured?

We established that the heart is a muscle that contracts and relaxes similarly to any other muscle in the body. Just like other muscles in the body, the muscles contract due to an electrical signal that tells them when to contract and relax. The electrical signal, and subsequent contraction, can be measured by doing an electrocardiogram. The term electrocardiogram is composed of three parts: electro meaning electrical, cardio meaning heart, and gram meaning measurement or reading.

The waves indicate electrical activity and heart muscle contractions
Picture of electrocardiogram monitor

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