The Cardiac Cycle (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Naomi Holyoak

Updated on

The cardiac cycle

  • The cardiac cycle is the series of events that take place in one heart beat, including muscle contraction and relaxation

    • The contraction of the heart is called systole, while the relaxation of the heart is called diastole

  • Volume changes occur in the heart during the cardiac cycle

    • Contraction of the heart muscle causes a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes

  • Volume changes lead to corresponding pressure changes

    • When volume decreases, pressure increases

    • When volume increases, pressure decreases

The events of the cardiac cycle

Atrial systole

  • The walls of the atria contract

    • Atrial volume decreases

    • Atrial pressure increases 

  • The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

  • Blood is forced into the ventricles

    • There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

  • The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

Ventricular systole

  • The walls of the ventricles contract

    • Ventricular volume decreases

    • Ventricular pressure increases

  • The pressure in the ventricles rises above that in the atria

    • The AV valves are forced to close, preventing back flow of blood

  • The pressure in the ventricles rises above that in the aorta and pulmonary artery

    • The semilunar (SL) valves are forced open so blood is forced into the arteries and out of the heart

  • During this period:

    • the atria are relaxing; atrial diastole coincides with ventricular systole

    • the blood flow to the heart continues, so the relaxed atria begin to fill with blood again

Diastole

  • The ventricles and atria are both relaxed

  • The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

  • The atria continue to fill with blood  

    • Blood returns to the heart via the vena cava and pulmonary vein

  • Pressure in the atria rises above that in the ventricles, forcing the AV valves open

  • Blood flows passively into the ventricles without need for atrial systole

  • The cycle then begins again with atrial systole

Diagram illustrating the cardiac cycle: atrial systole, ventricular systole, and diastole, showing heart chambers contracting and relaxing.
The cardiac cycle is the events that occur while blood travels through the heart; note that this diagram does not show valve movement accurately

Valve movements during the cardiac cycle

  • Pressure changes cause the valves to open and close in different regions of the heart

    • Valves open when the pressure of blood behind them is greater than the pressure in front of them

    • They close when the pressure of blood in front of them is greater than the pressure behind them

Stage in cardiac cycle

Atrioventricular valves

Semilunar valves

Atrial systole

Open

Closed

Ventricular systole

Closed

Open

Diastole

Open

Closed

Analysing data relating to the cardiac cycle

  • The changes that occur during the cardiac cycle can be shown on a graph, e.g.:

    • The lines on the graph below represent the pressure of the left atrium, aorta, and the left ventricle

Graph showing heart cycle phases: atrial systole, ventricular systole, and diastole. Pressure (kPa) vs time (seconds) for aorta, left ventricle, left atrium.
The pressure changes taking place during the cardiac cycle can be shown on a graph

Interpreting the cardiac cycle graph

  • Point A: the end of diastole

    • The atrium has filled with blood during the preceding diastole

    • Pressure is higher in the atrium than in the ventricle, so the AV valve is open

  • Point A-B: atrial systole

    • Left atrium contracts, causing an increase in atrial pressure and forcing blood into the left ventricle

    • Ventricular pressure increases slightly as it fills with blood

    • Pressure is higher in the atrium than in the ventricle, so the AV valve is open

  • Point B: beginning of ventricular systole

    • Left ventricle contracts causing the ventricular pressure to increase

    • Pressure in the left atrium drops as the muscle relaxes

    • Pressure in the ventricle exceeds pressure in the atrium, so the AV valve shuts

  • Point C: ventricular systole

    • The ventricle continues to contract

    • Pressure in the left ventricle exceeds that in the aorta

    • Aortic valve opens and blood is forced into the aorta

  • Point D: beginning of diastole

    • Left ventricle has been emptied of blood

    • Muscles in the walls of the left ventricle relax and pressure falls below that in the newly filled aorta

    • Aortic valve closes

  • Point D-E: early diastole

    • The ventricle remains relaxed and ventricular pressure continues to decrease

    • In the meantime, blood is flowing into the relaxed atrium from the pulmonary vein, causing an increase in pressure

  • Point E: diastole

    • The relaxed left atrium fills with blood, causing the pressure in the atrium to exceed that in the newly emptied ventricle

    • AV valve opens

  • After point E: late diastole

    • There is a short period of time during which the left ventricle expands due to relaxing muscles

    • This increases the internal volume of the left ventricle and decreases the ventricular pressure

    • At the same time, blood is flowing slowly through the newly opened AV valve into the left ventricle, causing a brief decrease in pressure in the left atrium

    • The pressure in both the atrium and ventricle then increases slowly as they continue to fill with blood

Worked Example

The graph below shows the cardiac cycle.

Calculate the heart rate of this person. Give your answer in beats per minute.

cardiac-cycle-calculation

Answer:

Step 1: work out the length of one heart beat

  • It takes 0.7 seconds for completion of one cardiac cycle, which is one heart beat 

1 cycle = 0.7 seconds

Step 2: calculate how many heart beats occur per second

  • Determine the number of times 0.7 fits into 1 second

divided by 0.7 = 1.43 beats per second

Step 3: calculate how many heart beats occur per minute

  • 1 minute = 60 seconds

1.43 x 60 = 85.71 beats beats / min

Examiner Tips and Tricks

Make sure that you can point out the times on a cardiac cycle graph at which different valves are opening and closing.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Naomi Holyoak

Reviewer: Naomi Holyoak

Expertise: Biology Content Creator

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.