How the Heart Works

The heart is an organ about the size of your fist that pumps blood through your body. It is made up of multiple layers of tissue.

Your heart is at the center of your circulatory system. This system is a network of blood vessels, such as arteries, veins, and capillaries, that carries blood to and from all areas of your body. Your blood carries the oxygen and nutrients that your organs need to work properly. Blood also carries carbon dioxide to your lungs so you can breathe it out. Inside your heart, valves keep blood flowing in the right direction.

Your heart’s electrical system controls the rate and rhythm of your heartbeat. A healthy heart supplies your body with the right amount of blood at the rate needed to work well. If disease or injury weakens your heart, your body’s organs will not receive enough blood to work normally. A problem with the electrical system—or the nervous or endocrine systems, which control your heart rate and blood pressure—can also make it harder for the heart to pump blood.

Explore this Health Topic to learn more about how the heart works, our role in research to improve health, and where to find more information.

Anatomy of Your Heart - How the Heart Works

Your heart is in the center of your chest, near your lungs. It has four hollow heart chambers surrounded by muscle and other heart tissue. The chambers are separated by heart valves, which make sure that the blood keeps flowing in the right direction. Read more about heart valves in Blood Flow.

Anatomy of the interior of the heart.
Anatomy of the interior of the heart. This image shows the four chambers of the heart and the direction that blood flows through the heart. Oxygen-poor blood, shown in blue-purple, flows into the heart and is pumped out to the lungs. Then oxygen-rich blood, shown in red, is pumped out to the rest of the body, with the help of the heart valves. Medical Illustration Copyright © 2019 Nucleus Medical Media, All rights reserved.


Heart chambers
- How the Heart Works

The two upper chambers of your heart are called atria, and the two lower chambers are called ventricles. Blood flows from the body and lungs to the atria and from the atria to the ventricles. The ventricles pump blood out of the heart to the lungs and other parts of the body. An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum.

Chambers of the heart. Your heart has four chambers. Two upper chambers, called the left and right atria, and two lower chambers, called the left and right ventricles, contract in a steady rhythm known as your heartbeat. During a normal heartbeat, blood from your tissues and lungs flow into your atria, then into your ventricles. Walls inside your heart, called the interatrial septum and intraventricular septum, help keep the blood on both sides from mixing. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Heart tissue
- How the Heart Works

The heart is made of three layers of tissue.

  • Endocardium, the thin inner lining of the heart chambers that also forms the surface of the valves.
  • Myocardium, the thick middle layer of muscle that allows your heart chambers to contract and relax to pump blood to your body.
  • Pericardium, the sac that surrounds your heart. Made of thin layers of tissue, it holds the heart in place and protects it. A small amount of fluid between the layers helps reduce friction between the beating heart and surrounding tissues.

Heart muscle. In order to pump blood more efficiently, your heart muscle, called myocardium, is arranged in a unique pattern. Three layers of myocardium wrap around the lower part of your heart. They twist and tighten in different directions to push blood through your heart. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Some conditions can affect the heart's tissue. Examples include:

  • Cardiomyopathy, in which the heart muscle becomes enlarged, thick, or rigid. As cardiomyopathy worsens, the heart becomes weaker and is less able to pump blood through the body and maintain a normal electrical rhythm.
  • Heart inflammation, which is inflammation in one or more of the layers of tissue in the heart, including the pericardium, myocardium, or endocardium. This can lead to serious complications, including heart failure, cardiogenic shock, or irregular heart rhythm.

How does the heart form in the womb?

Blood Flow - How the Heart Works

Arteries and veins link your heart to the rest of the circulatory system. Veins bring blood to your heart. Arteries take blood away from your heart. Your heart valves help control the direction the blood flows.

Heart valves
- How the Heart Works

Heart valves control the flow of blood so that it moves in the right direction. The valves prevent blood from flowing backward.

The heart has four valves.

  • The tricuspid valve separates the right atrium and right ventricle.
  • The mitral valve separates the left atrium and left ventricle.
  • The pulmonary valve separates the right ventricle and the pulmonary artery.
  • The aortic valve separates the left ventricle and aorta.

The valves open and shut in time with the pumping action of your heart's atria and ventricles. The opening and closing involves a set of flaps called cusps or leaflets. The cusps open to allow blood to flow out of a chamber and close to allow the chamber to refill with blood. Heart valve diseases can cause backflow or slow the flow of blood through the heart.

Valves of the heart. Two valves sit like doors between your atria and ventricles to prevent blood from flowing backward into your atria. The tricuspid valve opens into your right ventricle, and the mitral valve opens into your left ventricle. Strong thin tissues called chordae tendineae hold your valves in place during the forceful contractions of your ventricles. Blood leaving the ventricles passes through another set of valves, the pulmonary valve, between your right ventricle and pulmonary trunk, and the aortic valve, connecting your left ventricle and aorta. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Adding oxygen to blood
- How the Heart Works

Oxygen-poor blood from the body enters your heart through two large veins called the superior and inferior vena cava. The blood enters the heart's right atrium and is pumped to your right ventricle, which in turn pumps the blood to your lungs.

The pulmonary artery then carries the oxygen-poor blood from your heart to the lungs. Your lungs add oxygen to your blood. The oxygen-rich blood returns to your heart through the pulmonary veins. Visit our How the Lungs Work Health Topic to learn more about what happens to the blood in the lungs.

The oxygen-rich blood from the lungs then enters the left atrium and is pumped to the left ventricle. The left ventricle generates the high pressure needed to pump the blood to your whole body through your blood vessels.

When blood leaves the heart to go to the rest of the body, it travels through a large artery called the aorta. A balloon-like bulge, called an aortic aneurysm, can sometimes occur in the aorta.

Circulation and the heart. Your heart is divided into left and right halves, which work together like a dual pump. On the right side of your heart, oxygen-poor blood from your body’s tissues flows through large veins, called the superior and inferior vena cava, into your right atrium. Next, the blood moves into your right ventricle, which contracts and sends blood out of your heart to your lungs, to gather oxygen and get rid of carbon dioxide. On the left side of your heart, oxygen-rich blood from your lungs flows through your pulmonary veins into your left atrium. The blood then moves into your left ventricle, which contracts and sends blood out of your heart through the aorta to feed your cells and tissues. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Supplying oxygen to the heart’s muscle
- How the Heart Works

Like other muscles in the body, your heart needs blood to get oxygen and nutrients. Your coronary arteries supply blood to your heart. These arteries branch off from the aorta so that oxygen-rich blood is delivered to your heart as well as the rest of your body.

  • The left coronary artery delivers blood to the left side of your heart, including your left atrium and ventricle and the septum between the ventricles.
  • The circumflex artery branches off from the left coronary artery to supply blood to part of the left ventricle.
  • The left anterior descending artery also branches from the left coronary artery and provides blood to parts of both the right and left ventricles.
  • The right coronary artery provides blood to the right atrium and parts of both ventricles.
  • The marginal arteries branch from the right coronary artery and provide blood to the surface of the right atrium.
  • The posterior descending artery also branches from the right coronary artery and provides blood to the bottom of both ventricles.

Arteries supplying oxygen to the body. The coronary arteries branch off the aorta and supply the heart muscle with oxygen and nutrients. At the top of your aorta, arteries branch off to carry blood to your head and arms. Arteries branching from the middle and lower parts of your aorta supply blood to the rest of your body. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Some conditions can affect normal blood flow through these heart arteries. Examples include:

The coronary veins return oxygen-low blood from the heart's muscles back to the right atrium so it can be pumped to the lungs. They include:

  • The anterior cardiac veins.
  • The great cardiac vein.
  • The middle cardiac vein.
  • The small cardiac vein.

Your Heart's Electrical System - How the Heart Works

Your heartbeat is the contraction of your heart to pump blood to your lungs and the rest of your body. Your heart's electrical system determines how fast your heart beats.

Your heartbeat
- How the Heart Works

The contraction of the atria and ventricles makes a heartbeat. When your heart beats, it makes a “lub-DUB” sound. You may have heard this if you listened with a stethoscope or with your ear on someone's chest.

  • After your atria pump blood into the ventricles, the valves between the atria and ventricles close to prevent backflow. The “lub” is the sound of these valves closing.
  • After your ventricles contract to pump blood away from the heart, the aortic and pulmonary valves close and make the “DUB” sound.

Your heartbeat. Your heart beats an average of 60 to 100 beats per minute. In that one minute, your heart pumps about five quarts of blood through your arteries, delivering a steady stream of oxygen and nutrients all over your body. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

What is my pulse, and how do I measure it?

Electrical activity
- How the Heart Works

Electrical signals cause muscles to contract. Your heart has a special electrical system called the cardiac conduction system. This system controls the rate and rhythm of the heartbeat.

With each heartbeat, an electrical signal travels from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood. The heartbeat process includes the following steps.

  • The signal begins in a group of cells, called pacemaker cells, located in the sinoatrial (SA) node in the right atrium.
  • The electrical signal travels through the atria, causing them to pump blood into the ventricles.
  • The electrical signal then moves down to a group of pacemaker cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here the signal slows down slightly, allowing the ventricles time to finish filling with blood.
  • The AV node fires another signal that travels along the walls of your ventricles, causing them to contract and pump blood out of your heart.
  • The ventricles relax, and the heartbeat process starts all over again in the SA node.

Your heart’s conduction system. When special cells called pacemaker cells generate electrical signals inside your heart, the heart muscle cells, called myocytes, contract as a group. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.

Some conditions affect the heart's electrical system. Examples include:

  • Arrhythmia, or an irregular heart rhythm. Atrial fibrillation is one of the most common types of arrhythmia.
  • Conduction disorders, in which electrical signals either do not generate properly, or they do not travel properly through the heart, or both.

How Your Body Controls Heart Rate and Blood Pressure - How the Heart Works

How fast and hard your heart beats is controlled by signals from your body’s nervous system, as well as by hormones from your endocrine system. These signals and hormones allow you to adapt to changes in the amount of oxygen and nutrients your body needs. For example, when you exercise, your muscles need more oxygen, so your heart beats faster. When you sleep, your heart beats slower.

Blood pressure
- How the Heart Works

Your blood pressure is the force of the blood pushing against the walls of your arteries as the heart pumps blood. It is made up of two numbers: systolic and diastolic.

  • Systolic pressure is the pressure when the ventricles pump blood out of the heart. The pressure on your arteries is highest during this time.
  • Diastolic pressure is the pressure between beats, when the heart is filling with blood. The pressure on your arteries is lowest during this time.

For most adults, healthy blood pressure is usually less than 120 over 80, which is written as your systolic pressure number over your diastolic pressure number.

Blood pressure of 120 (systolic) over 80 (diastolic) mm Hg

High blood pressure is what happens when blood flows through blood vessels at higher-than-normal pressures.

Autonomic nervous system
- How the Heart Works

Your heart rate is controlled by the autonomic nervous system, also called the involuntary nervous system because it happens without your thinking about it. There are two opposing effects of the autonomic nervous system on your heart.

  • The parasympathetic system tells your heart to beat slower during rest.
  • The sympathetic system tells your heart to beat faster. This is the “fight-or-flight response.” When activated, it releases a chemical signal called norepinephrine that causes the heart to beat faster. Norepinephrine also signals the muscle in your heart to beat harder.

In a healthy person, the heart rate reflects a balance between these two systems.

Endocrine system
- How the Heart Works

A number of hormones from the endocrine system affect your heart and blood vessels.

Low levels of the hormone epinephrine, also called adrenaline, cause blood vessels to relax and widen. High levels of this same hormone, along with the hormone norepinephrine, cause the blood vessels to narrow and the heart rate to rise, increasing blood pressure.

Hormones also control how much water and salt your kidneys remove from your blood to excrete as urine. When your blood volume is low, such as when you are losing blood, certain hormones prevent water loss to help maintain your blood volume and blood pressure. The hormones also cause the blood vessels to narrow to maintain blood pressure. These hormones include:

  • The renin-angiotensin-aldosterone system, which can also cause the muscle cells in the heart to grow larger so they can pump harder.
  • Vasopressin, released from the pituitary gland.

Some hormones cause the kidneys to remove more water and salt from the blood. The decreased blood volume and salt cause your blood vessels to relax and lower your blood pressure. Atrial natriuretic peptide is a hormone made and released by heart cells when the pressure inside the atria is elevated.

The thyroid gland releases thyroid hormones that increase the heart rate. Problems with your thyroid gland can lead to heart problems such as an irregular heartbeat. Too much thyroid hormone can cause the heart to beat faster. Too little thyroid hormone can slow your heart rate.

Research for Your Health

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders, including heart conditions. Learn about current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research
- How the Heart Works

Learn about the following ways the NHLBI continues to translate current research and science into improved health for people who have heart conditions. Research on this topic is part of the NHLBI's broader commitment to advancing heart and vascular disease scientific discovery.

  • Long-standing Leader in Heart Research. For more than 70 years, the NHLBI has led the fight against heart and vascular diseases. During this period, steady, long-term investments in heart research have led to a greater understanding of how the heart works. These basic insights into the normal biology of the heart are essential for making biomedical discoveries that improve health for people who have heart and vascular diseases. In addition, research from the NHLBI's landmark effort, the Framingham Heart Study, has formed the basis of cardiovascular disease (CVD) prevention and health promotion guidelines and educational programs.
  • NHLBI Systematic Evidence Review of Lifestyle Interventions to Reduce Cardiovascular Risk. The NHLBI conducted a rigorous systematic review of evidence on the effect of dietary patterns, nutrient intake, and levels and types of physical activity on reducing CVD risk in adults. Results were incorporated into clinical guidelines for managing blood cholesterol and blood pressure in 2013. Visit Lifestyle Interventions to Reduce Cardiovascular Risk: Systematic Evidence Review from the Lifestyle Work Group for more information.
  • NHLBI Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents. We support the development of guidelines based on up-to-date research to evaluate and manage children and adolescents' risk of heart disease, including overweight and obesity. Visit Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents: Summary Report for more information.
  • Collaborating to Improve Women's Heart Disease Awareness. The Heart Truth® is a national education program for women that raises awareness about heart disease and its risk factors, including high blood pressure. It also educates and motivates women to take action to prevent the disease. The NHLBI sponsors The Heart Truth in partnership with many national and community organizations.
  • Studying Innovations to Improve Heart and Vascular Disease Outcomes. The Cardiothoracic Surgical Trials Network (CTSN) is an international clinical research enterprise that studies heart valve disease, arrhythmias, heart failure, coronary artery disease, and the complications of surgery. The CTSN's efforts extend from early translational research to the completion of six randomized clinical trials, three large observational studies, and multiple ancillary studies with more than 14,000 participants.

Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.

Advancing research for improved health
- How the Heart Works

In support of our mission, we are committed to advancing heart research, in part, through the following ways.

Learn about exciting research areas the NHLBI is exploring about the heart.

Participate in NHLBI Clinical Trials

We lead or sponsor many studies on the heart. See if you or someone you know is eligible to participate in our clinical trials.

Trials at the NIH Clinical Center

Heart health study of members of predominately black churches

This study aims to understand the health and health needs of people in predominately black churches in Washington, D.C. This information will help researchers design programs to improve heart health in these communities. To participate in this study, you must be between 19 and 85 years old and attend one of the churches in the study. This study is based in Bethesda, Maryland.

Cardiac magnetic resonance imaging (MRI) study

This study aims to improve cardiac magnetic resonance imaging (MRI) techniques. The clinical investigators will use new MRI methods to look at healthy people and people who have heart problems. Participants in this study must be at least 7 years old, be healthy or have cardiovascular disease, and not be pregnant. This study is located in Bethesda, Maryland.

To learn more about clinical trials at the NIH Clinical Center or to talk to someone about a study that might fit your needs, call the Office of Patient Recruitment 800-411-1222.

Are you interested in helping researchers understand what causes heart failure?

The study is using a technique called magnetic resonance spectroscopy (MRS) during a cardiac magnetic resonance imaging (MRI) scan to determine whether energy metabolism is disrupted in heart failure and whether this contributes to poor heart function. To participate in this study, you must be at least 18 years old and either be healthy or have coronary artery disease, dilated cardiomyopathy, or left ventricular hypertrophy. The study is located in Baltimore, Maryland.

Is your child undergoing heart surgery for congenital heart disease?

This study aims to help researchers better understand the developing heart and what controls the strength of its beats by comparing it to adult hearts. Researchers will study small pieces of the heart that are removed as a normal part of surgery or repair for children with congenital heart disease. To participate in this study, your child must be undergoing cardiopulmonary bypass surgery and be 18 years old or younger. The study is located in Atlanta, Georgia.

More Information

After reading our How the Heart Works Health Topic, you may be interested in additional information found in the following resources.


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