How the Heart Works How Blood Flows through the Heart

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

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 chambers. The opening and closing involve 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.

Adding oxygen to blood

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 page 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.

Supplying oxygen to the heart’s muscle

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.

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

• Angina
• Heart attack
• Coronary heart disease

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

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What the Heart Looks Like

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How the Heart Beats

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