Explore How the Heart Works
Your heart is located under your ribcage in the center of your chest between your right and left lungs. Its muscular walls beat, or contract, pumping blood to all parts of your body.
The size of your heart can vary depending on your age, size, and the condition of your heart. A normal, healthy, adult heart usually is the size of an average clenched adult fist. Some diseases can cause the heart to enlarge.
Below is a picture of the outside of a normal, healthy, human heart.
In figure B, the heart is the muscle in the lower half of the picture. The heart has four chambers. The heart's upper chambers, the right and left atria (AY-tree-uh), are shown in purple. The heart's lower chambers, the right and left ventricles (VEN-trih-kuls), are shown in red.
Some of the main blood vessels (arteries and veins) that make up your circulatory system are directly connected to the heart.
In figure B above, the superior and inferior vena cavae are shown in blue to the left of the heart muscle as you look at the picture. These veins are the largest veins in your body.
After your body's organs and tissues have used the oxygen in your blood, the vena cavae carry the oxygen-poor blood back to the right atrium of your heart.
The superior vena cava carries oxygen-poor blood from the upper parts of your body, including your head, chest, arms, and neck. The inferior vena cava carries oxygen-poor blood from the lower parts of your body.
The oxygen-poor blood from the vena cavae flows into your heart's right atrium and then to the right ventricle. From the right ventricle, the blood is pumped through the pulmonary (PULL-mun-ary) arteries (shown in blue in the center of figure B) to your lungs.
Once in the lungs, the blood travels through many small, thin blood vessels called capillaries. There, the blood picks up more oxygen and transfers carbon dioxide to the lungs—a process called gas exchange. To learn more about gas exchange, go to the Health Topics How the Lungs Work article.
The oxygen-rich blood passes from your lungs back to your heart through the pulmonary veins (shown in red to the left of the right atrium in figure B).
Oxygen-rich blood from your lungs passes through the pulmonary veins (shown in red to the right of the left atrium in figure B above). The blood enters the left atrium and is pumped into the left ventricle.
From the left ventricle, the oxygen-rich blood is pumped to the rest of your body through the aorta. The aorta is the main artery that carries oxygen-rich blood to your body.
Like all of your organs, your heart needs oxygen-rich blood. As blood is pumped out of your heart's left ventricle, some of it flows into the coronary arteries (shown in red in figure B).
Your coronary arteries are located on your heart's surface at the beginning of the aorta. They carry oxygen-rich blood to all parts of your heart.
Below is a picture of the inside of a normal, healthy, human heart.
Figure B shows the inside of your heart and how it's divided into four chambers. The two upper chambers of your heart are called the atria. They receive and collect blood.
The two lower chambers of your heart are called ventricles. The ventricles pump blood out of your heart to other parts of your body.
An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum.
The area of the septum that divides the atria is called the atrial or interatrial septum. The area of the septum that divides the ventricles is called the ventricular or interventricular septum.
Figure B 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.
The arrows in figure B show the direction that blood flows through your heart. The light blue arrow shows 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 arrow shows oxygen-rich blood coming 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. The left ventricle pumps the blood to the rest of your body through the aorta.
For the heart to work well, your blood must flow in only one direction. Your heart's valves make this possible. Both of your heart's ventricles have an "in" (inlet) valve from the atria and an "out" (outlet) valve leading to your arteries.
Healthy valves open and close in exact coordination with the pumping action of your heart's atria and ventricles. Each valve has a set of flaps called leaflets or cusps that seal or open the valve. This allows blood to pass through the chambers and into your arteries without backing up or flowing backward.
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The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.