The human heart is a muscle about the size of your fist that is located behind and slightly to the left of the breastbone. Its function is to pump blood throughout the body to provide oxygen and nutrients, remove waste products, pump deoxygenated blood through the lungs to release carbon dioxide and collect oxygen. Every minute, it pumps 5L of blood around the body.

The Heart's Structure

The heart has four chambers:

• Two smaller, upper chambers (the right and left atria)
• Two larger, lower pumping chambers (the right and left ventricles)
  

The upper and lower chambers are separated by one-way valves that ensure the forward motion of blood. The heart is separated into right and left sides, with each side performing a unique function:

• The right side of the heart receives deoxygenated blood from the body and sends it to the lungs to pick up oxygen.
• The left side of the heart receives oxygenated blood from the lungs and sends it to the rest of the body where the oxygen is delivered to organs such as the brain and kidneys.

Electrical Pathways within the Heart

In a healthy heart, each heartbeat begins in the sinus node (the heart's natural pacemaker), which is located in the right atrium. The electrical signal from the sinus node (sinoatrial or SA node) starts an electrical chain reaction that spreads across both atria. This causes the atria to contract and pump blood into the ventricles.

This electrical chain reaction continues from the atria through an area between the atria and ventricles called the atrioventricular (AV) node or junction. The AV node connects to conduction pathways that relay the electrical signal to both ventricles. The AV node acts as an electrical gateway to the ventricles. The conduction pathways deliver the signal, which causes the ventricles to contract and pump blood to the body.

An irregularity in the heart's electrical system is called an arrhythmia, or heart rhythm disorder. Rhythm disorders can cause the heart to beat too slowly (bradycardia), too fast (tachycardia), or not in sequence (asynchrony). An arrhythmia can compromise the heart's ability to pump and circulate blood through the body and brain.

The Heart's Function

A healthy heart is very sensitive to the body's needs. It adjusts its rate of pumping blood to match the body's need for oxygen and nutrients. An average person has approximately 120,000 heartbeats a day.

A normal heart rate can range from 60-140 beats per minute depending upon age and whether the person is at rest or exercising. The younger the person, the faster the heart rate. As you exercise or perform daily activities your heart increases its rate to ensure that it is supplying enough oxygen to your muscles to meet the needs of the activity. In a matter of seconds the heart can increase its pumping volume up to four times the amount that it pumps when at rest.

The pumping efficiency of the heart is evaluated using a measurement called ejection fraction (EF). The ejection fraction of the heart is defined as the fraction or proportion of blood that the heart pumps (or ejects) out of the left ventricular chamber with each heart beat. People with a normal healthy heart have an ejection fraction of 55 percent or greater. This means that 55 percent of the blood that fills the left ventricle is pumped out to the body with each contraction.

A low ejection fraction (EF) is one of the key indicators used to identify heart failure patients. A below-normal EF means your heart is no longer pumping efficiently and therefore may not be able to supply your organs and the rest of your body with enough blood. If your EF is low you may need further tests to determine if you are at risk of complications such as abnormal heart rhythms.

Last updated: 20 January 2005