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Your heart is divided into four chambers. The two chambers at the top are called the left atrium and the right atrium, and the two at the bottom are called the left ventricle and the right ventricle.

Every time your heart beats, several things happen.

• Your heart relaxes so blood can get into the two upper chambers
• The two upper chambers then get smaller, squeezing blood into the two lower chambers
• The two lower chambers then get smaller and pump blood out to your lungs and the rest of your body
• Veins throughout the body return blood back to your heart and the cycle begins again.

Normally, your heart beats about 60 to 80 times a minute. But it can beat as many as 150 times a minute if you are exercising hard and your body needs more oxygen.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.