We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

What is the Cardiac Cycle?

By Caitlin Kenney
Updated Feb 25, 2024
Our promise to you
WiseGeek is dedicated to creating trustworthy, high-quality content that always prioritizes transparency, integrity, and inclusivity above all else. Our ensure that our content creation and review process includes rigorous fact-checking, evidence-based, and continual updates to ensure accuracy and reliability.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

Editorial Standards

At WiseGeek, we are committed to creating content that you can trust. Our editorial process is designed to ensure that every piece of content we publish is accurate, reliable, and informative.

Our team of experienced writers and editors follows a strict set of guidelines to ensure the highest quality content. We conduct thorough research, fact-check all information, and rely on credible sources to back up our claims. Our content is reviewed by subject-matter experts to ensure accuracy and clarity.

We believe in transparency and maintain editorial independence from our advertisers. Our team does not receive direct compensation from advertisers, allowing us to create unbiased content that prioritizes your interests.

The cardiac cycle is the sequence of pumping and filling that occurs from the start of a heartbeat to its finish. The heart undergoes two main events during a complete cardiac cycle, diastole and systole. During diastole, the heart relaxes and fills with blood and during systole, the heart contracts and pumps the blood. It takes about one second to complete an entire cardiac cycle.

The heart is composed of four chambers: the two atria on top and two ventricles on bottom. The atria receive blood from the body into the heart and the ventricles are responsible for pumping blood out of the heart. The first stage is a systolic, or relaxed, stage. The right atrium takes in deoxygenated blood from the body through the superior and inferior vena cava. Since the right atrium and ventricle are relaxed and the atrioventricular valve between the two chambers is open, the blood flows into the right ventricle.

An electrical impulse from the sinoatrial (SA) node tells the atrium to contract and push in the remaining blood and signals to the Purkinje fibers. The Purkinje fibers cause the ventricle to contract, or go into the first stage of systole in the cardiac cycle, and pump the deoxygenated blood out of a semilunar valve into the pulmonary artery. The pulmonary artery takes the blood to the lungs, where it can be reoxygenated.

From the lungs, the oxygenated blood comes back into the heart via the pulmonary veins, and is received in the left atrium. The heart is now in the second stage of diastole, so the left atrium is relaxed and the atrioventricular valve is open. The blood passes though the valve into the left ventricle. The left ventricle then gets the message from the Purkinje fibers to contract, or go into the second stage of systole. This contraction forces open the semilunar valve to the aorta, where the blood is pumped out to the rest of the body.

Because blood is circulating continuously, the heart doesn’t have to wait for blood to return from the body or the lungs, so it is constantly filling up and pumping out. For this reason, the filling up of one side of the heart occurs at the same time as the filling up of other side, and the contraction of one side occurs simultaneously with the contraction of the other. So the first and second stages of diastole in the cardiac cycle happen simultaneously, and then the first and second stages of systole occur. The “lub-dup” noise heard when listening to the heartbeat is the sound of the valves closing. The “lub” is the sound of the atrioventricular valves closing, and the “dup” is the sound of the aortic valve closing.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
WiseGeek, in your inbox

Our latest articles, guides, and more, delivered daily.

WiseGeek, in your inbox

Our latest articles, guides, and more, delivered daily.