The cardiac cycle
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of heart structure and the flow of blood through the chambers and valves. You can test your knowledge on these below.
What are the functions of the heart valves?
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Heart valves prevent the backflow of blood and ensure unidirectional flow through the heart chambers.
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The atrioventricular (AV) valves prevent backflow into the atria when the ventricles contract.
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The semilunar valves prevent backflow into the ventricles from the arteries
What is the function of the septum in the heart?
The septum separates the left and right sides of the heart to prevent the mixing of oxygenated and deoxygenated blood.
Why does the left ventricle have a thicker wall than the right ventricle?
The left ventricle pumps blood to the entire body under high pressure, whereas the right ventricle pumps blood only to the lungs under lower pressure.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains cardiac cycle or read the full notes below. Once you've gone through the whole note, try out the practice questions!
The Cardiac Cycle Overview
The cardiac cycle describes the sequence of events in one heartbeat, consisting of three main phases:
- Atrial Systole – The atria contract, forcing blood into the ventricles.
- Ventricular Systole – The ventricles contract, pumping blood into the arteries.
- Diastole – All chambers of the heart relax, allowing blood to flow into the atria.
Heart Valve Function
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Heart valves ensure one-way flow of blood by opening and closing in response to pressure changes within the heart chambers. When the pressure behind the valve is greater, the valve opens. When the pressure in front is greater the valves closes.
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Atrioventricular (AV) Valves:
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Open during atrial systole, allowing blood to flow into the ventricles.
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Close during ventricular systole to prevent backflow into the atria.
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Semilunar Valves (Pulmonary & Aortic):
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Open during ventricular systole, allowing blood to be ejected from the ventricles into the arteries.
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Close during diastole to prevent blood from flowing back into the ventricles from the arteries.
Phases of the Cardiac Cycle
1. Atrial Systole (Contraction of the Atria)
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The atria contract, decreasing the volume and increasing pressure and forcing blood through the AV valves into the ventricles.
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The AV valves are open as pressure is higher in the atria than in the ventricles.
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The semilunar valves remain closed to prevent blood from flowing into the arteries.
2. Ventricular Systole (Contraction of the Ventricles)
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The ventricles contract, increasing pressure and forcing blood through the semilunar valves into the pulmonary artery and aorta.
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The semilunar valves are open as pressure is higher in the ventricles than in the arteries.
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The AV valves are closed, preventing backflow into the atria, as the pressure in the ventricles is higher than the pressure in the atria.
3. Diastole (Relaxation Phase)
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The heart muscle relaxes, and pressure in the atria and ventricles decreases.
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The semilunar valves close to prevent backflow into the ventricles from the arteries.
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The AV valves open, allowing blood to passively fill the atria and ventricles. This leads to a small increase in pressure.
Analysing a Cardiac Cycle Graph
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The cardiac cycle graph shows pressure changes in the atria, ventricles, and aorta (the artery carrying blood from the left ventricle).
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Key points:
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Where lines cross over is where the valves open or close.
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A: Atrial systole: A small increase in atrial pressure as the atria contract.
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B: Ventricular systole: A sharp increase in ventricular pressure as the ventricles contract, the Atrioventricular valves close as pressure in the ventricles now exceeds pressure in the atria.
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C: Ventricular pressure exceeds aortic pressure so semi lunar valves open. The pressure then decreases as the ventricles empty.
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D: Diastole: A drop in pressure as the ventricles relax. The semilunar valves close as pressure in the aorta now exceeds pressure in the ventricles.
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E: Atrioventricular valves open as pressure in the atria exceeds pressure in the ventricles. Atrial and ventricular filling as blood enters the chambers.
Exam Tip
Look for the closure of AV valves and semilunar valves on the graph to identify the stages of the cycle.
The right side of the heart would have the same shape graph but a lower maximum pressure (as the right ventricle has a thinner muscular wall).
No answer provided.
Calculating Heart Rate from a Graph
- Heart rate is calculated using the formula:
- Heart Rate (bpm) = 60 / Time for one cardiac cycle (s)
- You can identify the duration of one full cardiac cycle from the graph (e.g. peak-to-peak of each ventricular contraction).
Exam Tip
Be able to label and interpret a cardiac cycle graph, and calculate heart rate from it.
No answer provided.
Key Terms
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Systole: Contraction phase of the heart.
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Diastole: Relaxation phase of the heart.
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Atrioventricular (AV) valves: Prevent backflow into the atria (tricuspid and bicuspid valves).
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Semilunar valves: Prevent backflow into the ventricles (aortic and pulmonary valves).
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Cardiac cycle: One complete heartbeat.
No answer provided.
The image below shows heart valves during one stage of a cardiac cycle.
Ventricles are visible through the open valves.
What can you conclude from the appearance of valves in the image above about heart muscle activity and blood movement between:
- Ventricles and arteries? (2 marks)
- Atria and ventricles? (2 marks)
Ventricles and arteries
1. Ventricle relaxed / Diastole
2. No blood backflow into ventricles from arteries
(We know this because we can see the semi-lunar valves are closed on the diagram)
Atria and ventricles
3. Atria contracted / atrial systole
4. Blood movement from atria into ventricles
(We know this because we can see the atrioventricular valves are open on the diagram)
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!