Grade 12 · Biology
The heart & cardiac cycle
Why it matters
Structure and the heartbeat.
Advanced20m readingPrerequisite: None
What is the cardiac cycle?
Short answer
The cardiac cycle is one complete heartbeat, the sequence of contraction and relaxation that pumps blood. During systole the heart muscle contracts and forces blood out of the chambers; during diastole it relaxes and refills. The atria contract first to fill the ventricles, then the ventricles contract to eject blood to the lungs and body, with valves preventing backflow.
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Frequently asked questions
- What is the difference between systole and diastole?
- Systole is the contraction phase that pumps blood out; diastole is the relaxation phase that lets the chambers refill.
- What do the heart valves do?
- They ensure one-way flow, opening to let blood through and closing to prevent it flowing backward between beats.