What Is the Heart?

Your heart is a muscular organ that acts like a pump to send blood throughout your body all the time.

Your heart is at the center of your circulatory system, which delivers blood to all areas of your body. An electrical system regulates the heart and uses electrical signals to contract the heart’s walls. When the walls contract, blood is pumped into your circulatory system.

Your circulatory system is made up of a network of blood vessels, such as arteries, veins, and capillaries. The vessels in this network carry blood to and from all areas of your body. A system of inlet and outlet valves in your heart’s chambers works to ensure that blood flows in the right direction.

Your heart is vital to your health and nearly everything that goes on in your body. Without the heart’s pumping action, blood can’t circulate within your body.

Your blood carries the oxygen and nutrients that your organs need to function normally. Blood also carries carbon dioxide, a waste product, to your lungs to be passed out of your body and into the air. A healthy heart supplies the areas of your body with the right amount of blood at the right rate needed to function normally. If disease or injury weakens your heart, your body’s organs won’t receive enough blood to function normally.

Anatomy of the Heart

Your heart is located under the ribcage in the center of your chest between your right and left lung. It’s shaped like an upside-down pear. Its muscular walls beat, or contract, pumping blood continuously to all parts of your body.

The size of your heart can vary depending on your age, size, or the condition of your heart. A normal, healthy, adult heart most often is the size of an average clenched adult fist. Some diseases of the heart can cause it to become larger.

The Exterior of the Heart

Below is a picture of the outside of a normal, healthy, human heart.

Heart Exterior

The illustration shows the front surface of the heart, including the coronary arteries and major blood vessels.

The heart is the muscle in the lower half of the picture. The heart has four chambers. The right and left atria (AY-tree-uh) are shown in purple. The right and left ventricles (VEN-trih-kuls) are shown in red.

Connected to the heart are some of the main blood vessels—arteries and veins—that make up your blood circulatory system.

The ventricle on the right side of your heart pumps blood from the heart to your lungs. When you breathe air in, oxygen passes from your lungs through blood vessels where it’s added to your blood. Carbon dioxide, a waste product, is passed from your blood through blood vessels to your lungs and is removed from your body when you breathe air out.

The atrium on the left side of your heart receives oxygen-rich blood from the lungs. The pumping action of your left ventricle sends this oxygen-rich blood through the aorta (a main artery) to the rest of your body.

The Right Side of Your Heart

The superior and inferior vena cavae are in blue to the left of the muscle as you look at the picture. These veins are the largest veins in your body. They carry used (oxygen-poor) blood to the right atrium of your heart. “Used” blood has had its oxygen removed and used by your body’s organs and tissues. The superior vena cava carries used blood from the upper parts of your body, including your head, chest, arms, and neck. The inferior vena cava carries used blood from the lower parts of your body.

The used blood from the vena cavae flows into your heart’s right atrium and then on to the right ventricle. From the right ventricle, the used blood is pumped through the pulmonary (PULL-mun-ary) arteries (in blue in the center of picture) to your lungs. Here, through many small, thin blood vessels called capillaries, your blood picks up oxygen needed by all the areas of your body.

The oxygen-rich blood passes from your lungs back to your heart through the pulmonary veins (in red to the left of the right atrium in the picture).

The Left Side of Your Heart

Oxygen-rich blood from your lungs passes through the pulmonary veins (in red to the right of the left atrium in the picture). It enters the left atrium and is pumped into the left ventricle. From the left ventricle, your blood is pumped to the rest of your body through the aorta.

Like all of your organs, your heart needs blood rich with oxygen. This oxygen is supplied through the coronary arteries as it’s pumped out of your heart’s left ventricle. Your coronary arteries are located on your heart’s surface at the beginning of the aorta. Your coronary arteries (shown in red in the drawing) carry oxygen-rich blood to all parts of your heart.

The Interior of the Heart

Below is a picture of the inside of a normal, healthy, human heart.

Heart Interior

The illustration shows a cross-section of a healthy heart and its inside structures. The blue arrow shows the direction in which low-oxygen blood flows from the body to the lungs. The red arrow shows the direction in which oxygen-rich blood flows from the lungs to the rest of the body.

The Septum

The right and left sides of your heart are divided by an internal wall of tissue called the septum. The area of the septum that divides the two upper chambers (atria) of your heart is called the atrial or interatrial septum. The area of the septum that divides the two lower chambers (ventricles) of your heart is called the ventricular or interventricular septum.

Heart Chambers

The picture shows the inside of your heart and how it’s divided into four chambers. The two upper chambers of your heart are called atria. The atria receive and collect blood. The two lower chambers of your heart are called ventricles. The ventricles pump blood out of your heart into the circulatory system to other parts of your body.

Heart Valves

The picture shows your heart’s four valves. Shown counterclockwise in the picture, the valves include the aortic (ay-OR-tik) valve, the tricuspid (tri-CUSS-pid) valve, the pulmonary valve, and the mitral (MI-trul) valve.

Blood Flow

The arrows in the drawing show the direction that blood flows through your heart. The light blue arrows show that blood enters the right atrium of your heart from the superior and inferior vena cavae. From the right atrium, blood is pumped into the right ventricle. From the right ventricle, blood is pumped to your lungs through the pulmonary arteries.

The light red arrows show the oxygen-rich blood coming in from your lungs through the pulmonary veins into your heart’s left atrium. From the left atrium, the blood is pumped into the left ventricle, where it’s pumped to the rest of your body through the aorta.

For the heart to function properly, your blood flows in only one direction. Your heart’s valves make this possible. Both of your heart’s ventricles has an “in” (inlet) valve from the atria and an “out” (outlet) valve leading to your arteries. Healthy valves open and close in very exact coordination with the pumping action of your heart’s atria and ventricles. Each valve has a set of flaps called leaflets or cusps, which seal or open the valves. This allows pumped blood to pass through the chambers and into your arteries without backing up or flowing backward.

Heart Contraction and Blood Flow

The animation below shows how your heart pumps blood. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame of the animation. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how blood flows through the heart as it contracts and relaxes.

Heartbeat

Almost everyone has heard the real or recorded sound of a heartbeat. When your heart beats, it makes a "lub-DUB" sound. Between the time you hear "lub" and "DUB," blood is pumped through your heart and circulatory system.

A heartbeat may seem like a simple event repeated over and over. A heartbeat actually is a complicated series of very precise and coordinated events that take place inside and around your heart. Each side of your heart uses an inlet valve to help move blood between the atrium and ventricle. The tricuspid valve does this between the right atrium and ventricle. The mitral valve does this between the left atrium and ventricle. The "lub" is the sound of the mitral and tricuspid valves closing.

Each of your heart's ventricles has an outlet valve. The right ventricle uses the pulmonary valve to help move blood into the pulmonary arteries. The left ventricle uses the aortic valve to do the same for the aorta. The "DUB" is the sound of the aortic and pulmonary valves closing.

Each heartbeat has two basic parts: diastole (di-AS-toe-lee, or relaxation) and atrial and ventricular systole (SIS-toe-lee, or contraction). During diastole, the atria and ventricles of your heart relax and begin to fill with blood. At the end of diastole, your heart's atria contract (an event called atrial systole) and pump blood into the ventricles. The atria then begin to relax. Next, your heart's ventricles contract (an event called ventricular systole) and pump blood out of your heart.

Pumping Action

Your heart uses the four valves to ensure your blood flows only in one direction. Healthy valves open and close in coordination with the pumping action of your heart's atria and ventricles. Each valve has a set of flaps called leaflets or cusps. These seal or open the valves. This allows pumped blood to pass through the chambers and into your blood vessels without backing up or flowing backward.

Blood without oxygen from the two vena cavae fill your heart's right atrium. The atrium contracts (atrial systole). The tricuspid valve located between the right atrium and ventricle opens for a short time and then shuts. This allows blood to enter into the right ventricle without flowing back into the right atrium.

When your heart's right ventricle fills with blood, it contracts (ventricular systole). The pulmonary valve located between your right ventricle and pulmonary artery opens and closes quickly. This allows blood to enter into your pulmonary artery without flowing back into the right ventricle. This is important because the right ventricle begins to refill with more blood through the tricuspid valve. Blood travels through the pulmonary arteries to your lungs to pick up oxygen.

Oxygen-rich blood returns from the lungs to your heart's left atrium through the pulmonary veins. As your heart's left atrium fills with blood, it contracts. This event also is called atrial systole. The mitral valve located between the left atrium and left ventricle opens and closes quickly. This allows blood to pass from the left atrium into the left ventricle without flowing back into the left atrium.

As the left ventricle fills with blood, it contracts. This event also is called ventricular systole. The aortic valve located between the left ventricle and aorta opens and closes quickly. This allows blood to flow into the aorta. The aorta is the main artery that carries blood from your heart to the rest of your body. The aortic valve closes quickly to prevent blood from flowing back into the left ventricle, which is already filling up with new blood.

Circulation and Blood Vessels

Your heart and blood vessels make up your overall blood circulatory system. Your overall blood circulatory system is made up of four subsystems.

Arterial Circulation

Arterial circulation is that part of your overall blood circulatory system that involves arteries, like the aorta and pulmonary arteries.

Arteries are blood vessels that carry blood away from your heart. Healthy arteries are strong and elastic. They become narrow between beats of the heart, and they help keep your blood pressure consistent. This helps blood circulate efficiently through your body.

Arteries branch into smaller blood vessels called arterioles. Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to different parts of your body.

Venous Circulation

Venous circulation is the part of your overall blood circulatory system that involves veins, like the vena cavae and pulmonary veins. Veins are blood vessels that carry blood to your heart. Veins have thinner walls than arteries. Veins can increase in width as the amount of blood passing through them increases.

Capillary Circulation

Capillary circulation is the part of your circulatory system where oxygen, nutrients, and waste pass between your blood and parts of your body.

Capillaries connect the arterial and venous circulatory subsystems. Capillaries are very small blood vessels.

The importance of capillaries lies in their very thin walls. Unlike arteries and veins, capillary walls are thin enough that oxygen and nutrients in your blood can pass through the walls to the parts of your body that need them to function normally. Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood where it's taken away to your lungs.

Pulmonary Circulation

Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again. Pulmonary circulation includes both arterial and venous circulation.

Blood without oxygen is pumped to the lungs from the heart (arterial circulation). Oxygen-rich blood moves from the lungs to the heart through the pulmonary veins (venous circulation).

Pulmonary circulation also includes capillary circulation. Oxygen you breathe in from the air passes through your lungs into your blood through the many capillaries in the lungs. Oxygen-rich blood moves through your pulmonary veins to the left side of your heart and out the aorta to the rest of your body. Capillaries in the lungs also remove carbon dioxide from your blood so that your lungs can breathe the carbon dioxide out into the air.

Your Heart’s Electrical System

The animation below shows how your heart's electrical system works. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame of the animation. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how the heart's internal electrical conduction system causes the heart to pump blood.

Your heart’s electrical system controls all the events that occur when your heart pumps blood. The electrical system also is called the cardiac conduction system. If you’ve ever seen the heart test called an EKG (electrocardiogram), you’ve seen a graphical picture of the electrical activity of your heart.

Your heart’s electrical system is made up of three main parts:

• The sinoatrial (SA) node located in the right atrium of your heart
• The atrioventricular (AV) node located on the interatrial septum close to the tricuspid valve
• The His-Purkinje system located along the walls of your heart’s ventricles

A heartbeat is a complicated series of events that take place in your heart. A heartbeat is a single cycle in which your heart’s chambers relax and contract to pump blood. This cycle includes the opening and closing of the two inlet and outlet valves of the right and left ventricles of your heart.

Each heartbeat has two basic parts: diastole, and atrial and ventricular systole. During diastole, the atria and ventricles of your heart relax and begin to fill with blood. At the end of diastole, your heart’s atria contract (atrial systole), pumping blood into the ventricles, and then begin to relax. Your heart’s ventricles then contract (ventricular systole), pumping blood out of your heart.

Each beat of your heart is set in motion by an electrical signal from within your heart muscle. In a normal, healthy heart, each beat begins with a signal from the SA node. This is why the SA node is sometimes called your heart’s natural pacemaker. Your pulse, or heart rate, is the number of signals the SA node produces per minute.

The signal is generated as the two vena cavae fill your heart’s right atrium with blood from other parts of your body. The signal spreads across the cells of your heart’s right and left atria. This signal causes the atria to contract. This action pushes blood through the open valves from the atria into both ventricles.

The signal arrives at the AV node near the ventricles (see red burst on picture), where it slows for an instant to allow your heart’s right and left ventricles to fill with blood. The signal is released and moves to the His bundle located in the walls of your heart’s ventricles.

From the His bundle, the signal fibers divide into left and right bundle branches through the Purkinje fibers that connect directly to the cells in the walls of your heart’s left and right ventricles (see yellow on the picture). As the signal spreads across the cells of your heart’s ventricle walls, both ventricles contract, but not at exactly the same moment. The left ventricle contracts an instant before the right ventricle. This pushes blood through the pulmonary valve (for the right ventricle) to your lungs, and through the aortic valve (for the left ventricle) to the rest of your body.

As the signal passes, the walls of the ventricles relax and await the next signal.

This process continues over and over as the atria refill with blood and other electrical signals come from the SA node.

Heart Disease

Your heart is made up of many parts working together to pump blood. In a healthy heart, all the parts work well so that your heart pumps blood normally. Then all parts of your body that depend on the heart to deliver blood also stay healthy.

Heart disease can disrupt a heart's normal electrical system and pumping functions. Diseases and conditions of the heart's muscle make it difficult for your heart to pump blood normally. Damaged or diseased blood vessels make the heart work harder than normal. Problems with the heart's electrical system, called arrhythmias, can make it difficult for the heart to pump blood efficiently.