Summary
The sequence of atrial and ventricular contraction (systole) and relaxation (diastole) in each heartbeat.
In depth
Atrial systole fills ventricles; ventricular systole closes AV valves (first sound 'lub') and opens semilunar valves; diastole closes semilunar valves (second sound 'dub'). Cardiac output = heart rate × stroke volume, about 5 L per minute at rest.
Pressure and volume changes are shown in Wiggers diagrams. Heart sounds come from valve closure. Murmurs result from turbulent flow.
Example: stroke volume is about 70 mL.
One cycle at 75 beats per minute lasts about 0.8 seconds: atrial systole 0.1 s, ventricular systole 0.3 s, and diastole 0.4 s. Pressure changes drive valve movements: AV valves close when ventricular pressure exceeds atrial (S1, "lub"); semilunar valves open when ventricular pressure exceeds aortic pressure and close afterwards (S2, "dub") when the ventricles relax. Isovolumetric contraction and relaxation phases occur with all valves closed.
Stroke volume is about 70 mL, end-diastolic volume 120 mL, and ejection fraction about 55 to 70 percent. Cardiac output equals heart rate times stroke volume. Murmurs signal turbulent flow from valve problems. Wiggers diagrams show pressure, volume and ECG over time.
Key idea: the heart is two pumps working in series, and valves ensure one-way flow.
Frequently asked questions
What is Cardiac Cycle?
The sequence of atrial and ventricular contraction (systole) and relaxation (diastole) in each heartbeat.
Which area of biology covers Cardiac Cycle, and how advanced is it?
Cardiac Cycle is one of the 51 topics in Human Physiology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Heart Structure, Circulatory System, Blood Pressure.
Can you give an example of Cardiac Cycle?
Pressure curves show valves opening and closing.
What should I learn before Cardiac Cycle?
Work through the Beginner topics in Human Physiology first, such as Homeostasis, Tissues and Organs, Skeletal System, then return to Cardiac Cycle. Intermediate topics assume the Beginner vocabulary.
What topics are related to Cardiac Cycle?
The closest topics to Cardiac Cycle are Heart Structure, Circulatory System, Blood Pressure, Cardiac Conduction System. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Cardiac Cycle?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Cardiac Cycle. To practise the whole of Human Physiology, use the Human Physiology quiz.
How is Cardiac Cycle different from Heart Structure?
Cardiac Cycle: The sequence of atrial and ventricular contraction (systole) and relaxation (diastole) in each heartbeat. Heart Structure: A four-chambered muscular pump with atria, ventricles and valves that keep blood flowing one way. Read both side by side and note what each one explains.
How should I study Cardiac Cycle step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Heart Structure and explain the difference aloud. Finally, take the Human Physiology quiz and come back to this page for any question you miss.