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Every time your heart beats, three things happen.

• Your heart relaxes so blood can flow into it.
• The two upper chambers of your heart contract, squeezing blood into the two lower chambers.
• The two lower chambers contract and pump blood out. The right chamber pumps blood to your lungs. The left one pumps blood to the rest of your body.

Your heart beats about 60 times to 80 times a minute. But it can beat as many as 150 times a minute if you're exercising hard and your body needs more oxygen and food.

For more, see How blood moves through your heart and How your heart moves blood around your body.

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.