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How the Heart Works

Heart Contraction and Blood Flow

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, repeated event. However, it’s a complex series of very precise and coordinated events. These events 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 tricuspid and mitral valves closing.

Each of your heart’s ventricles also 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) and systole (SIS-toe-lee).

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) and pump blood into the ventricles.

The atria then begin to relax. Next, your heart’s ventricles contract (ventricular systole) and pump blood out of your heart.

Pumping Action

Your heart uses its four valves to ensure your blood flows in only 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 that seal or open the valve. The cusps allow pumped blood to pass through the chambers and into your blood vessels without backing up or flowing backward.

Oxygen-poor blood from the vena cavae fills 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 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 arteries 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 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 backward.

As the left ventricle fills with blood, it contracts. This event 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 already is filling up with new blood.

Taking Your Pulse

When your heart pumps blood through your arteries, it creates a pulse that you can feel on the arteries close to the skin’s surface. For example, you can feel the pulse on the artery inside of your wrist, below your thumb.

You can count how many times your heart beats by taking your pulse. You will need a watch with a second hand.

To find your pulse, gently place your index and middle fingers on the artery located on the inner wrist of either arm, below your thumb. You should feel a pulsing or tapping against your fingers.

Watch the second hand and count the number of pulses you feel in 30 seconds. Double that number to find out your heart rate or pulse for 1 minute.

The usual resting pulse for an adult is 60 to 100 beats per minute. To find your resting pulse, count your pulse after you have been sitting or resting quietly for at least 10 minutes.

Circulation and Blood Vessels

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

Arterial Circulation

Arterial circulation is the part of your circulatory system that involves arteries, like the aorta and pulmonary arteries. Arteries are blood vessels that carry blood away from your heart. (The exception is the coronary arteries, which supply your heart muscle with oxygen-rich blood.)

Healthy arteries are strong and elastic (stretchy). They become narrow between heartbeats, and they help keep your blood pressure consistent. This helps blood move through your body.

Arteries branch into smaller blood vessels called arterioles (ar-TEER-e-ols). Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to parts of your body.

Venous Circulation

Venous circulation is the part of your 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 widen 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 are very small blood vessels. They connect the arterial and venous circulatory subsystems.

The importance of capillaries lies in their very thin walls. Oxygen and nutrients in your blood can pass through the walls of the capillaries to the parts of your body that need them to work 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.

Oxygen-poor blood 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 of 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

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 heart’s electrical activity.

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 complex series of events. These events take place inside and around 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 inlet and outlet valves of the right and left ventricles of your heart.

Each heartbeat has two basic parts: diastole and 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) and pump blood into the ventricles. The atria 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 sometimes is 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 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. It slows for an instant to allow your heart’s right and left ventricles to fill with blood. The signal is released and moves along a pathway called the bundle of His, which is located in the walls of your heart’s ventricles.

From the bundle of His, the signal fibers divide into left and right bundle branches through the Purkinje fibers. These fibers connect directly to the cells in the walls of your heart’s left and right ventricles (see yellow on the picture in the animation).

The signal spreads across the cells of your ventricle walls, and both ventricles contract. However, this doesn’t happen 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 more 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. As a result, 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 hard for your heart to properly pump blood.

Damaged or diseased blood vessels make the heart work harder than normal. Problems with the heart’s electrical system, called arrhythmias (ah-RITH-me-ahs), can make it hard for the heart to pump blood efficiently.

Source-nhlbi.nih.gov