The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system.

• Each day the average heart beats (expands and contracts) 100,000 times and pumps about 2,000 gallons of blood.
• In a 70-year lifetime, an average human heart beats more than 2.5 billion times.

To understand how the heart pumps, learn about:

Structure of the heart
Electrical system of the heart
Abnormal heart rhythms (arrhythmias)

Structure of the heart: four chambers, four valves
The heart has four chambers, two on the right and two on the left:

• Two upper chambers arecalled atria (one is an atrium).
• Two lower chambers are called ventricles.

The heart also has four valves that open and close to let blood flow in only one direction when the heart contracts (beats). The four heart valves are:

• Tricuspid valve, located between the right atrium and right ventricle
• Pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery
• Mitral valve, between the left atrium and left ventricle
• Aortic valve, between the left ventricle and the aorta

Each valve has a set of flaps (also called leaflets or cusps). The mitral valve has two flaps; the others have three. Blood flow occurs only when there's a difference in pressure across the valves, which causes them to open. Under normal conditions, the valves permit blood to flow in only one direction.

The heart pumps blood to the lungs and to all the body's tissues by a sequence of highly organized contractions of the four chambers. For the heart to function properly, the four chambers must beat in an organized way.

Watch an animation of how the heart pumps blood 

Electrical signals control the pump
The heart beats (contracts) when an electrical impulse from the sinoatrial node (also called the SA node or sinus node) moves through it. The SA node is sometimes referred to as the heart's "natural pacemaker" because it initiates impulses for the heartbeat.

The normal electrical sequence begins in the right atrium and spreads throughout the atria to the atrioventricular (AV) node. From the AV node, electrical impulses travel down a group of specialized fibers called the His-Purkinje system to all parts of the ventricles.

This exact route must be followed for the heart to pump properly. As long as the electrical impulse is transmitted normally, the heart pumps and beats at a regular pace. In an adult, a normal heart beats 60 to 100 times a minute.

Electrocardiography (ECG or EKG) is a painless, non-invasive procedure that records the heart’s electrical activity and can help diagnose arrhythmias.

Watch an animation of the normal heart's electrical system
See a normal heart’s electrocardiogram (ECG)

Arrhythmias are abnormal beats
The term "arrhythmia" refers to any change from the normal sequence of electrical impulses, causing abnormal heart rhythms. Arrhythmias may be completely harmless or life-threatening.

Some arrhythmias are so brief (for example, a temporary pause or premature beat) that the overall heart rate or rhythm isn't greatly affected. But if arrhythmias last longer, they may cause the heart rate to be too slow or too fast or the heart rhythm to be erratic – so the heart pumps less effectively.

• A fast heart rate (in adults, more than 100 beats per minute) is called tachycardia.
• A slow heart rate (less than 60 beats per minute) is referred to as bradycardia.

Watch an animation of arrhythmias

This content is reviewed regularly. Last updated 02/18/09.