OCR AS Biology

Revision Notes

3.2.7 The Cardiac Cycle

The Cardiac Cycle

  • The contraction of the heart is called systole, while the relaxation of the heart is called diastole
  • Atrial systole is the period when the atria are contracting and ventricular systole is when the ventricles are contracting
  • Atrial systole happens around 0.13 seconds after ventricular systole
  • During ventricular systole, blood is forced out of the pulmonary artery (to the lungs) and aorta (to the rest of the body)
  • One systole and diastole makes a heartbeat and lasts around 0.8 seconds in humans. This is the cardiac cycle

Pressure changes

  • During systole and diastole, heart valves open and close as a result of pressure changes
  • Valves are an important mechanism to stop blood flowing backwards
  • During diastole, the heart is relaxing
    • The atrioventricular valves open and the semilunar valves are closed
  • During systole, the heart contracts and pushes blood out of the heart
    • During this time, the atrioventricular valves are closed and the semilunar valves are open

Pressure Changes in the Heart Table

Pressure Changes in the Heart Table, downloadable AS & A Level Biology revision notes

The cardiac cycle

  • A cardiac cycle is the sequence of events that make up a single heartbeat
  • It includes periods of heart muscle contraction and relaxation
  • One cardiac cycle is followed by another in a continuous process
    • There is no gap between cycles where blood stops flowing
  • The contraction of the muscles in the wall of the heart reduces the volume of the heart chambers and increases the pressure of the blood within that chamber
  • When the pressure within a chamber/vessel exceeds that in the next chamber/vessel the valves are forced open and the blood moves through
  • When the muscles in the wall of the heart relax they recoil which increases the volume of the chamber/vessel and decreases the pressure so that the valves close

Pressure Changes in the Cardiac Cycle, downloadable AS & A Level Biology revision notes

Analysing the cardiac cycle

  • There are several key points to keep in mind when analysing the cardiac cycle
  • The curves on the graph represent the pressure of the left atria, aorta and the left ventricle
  • The points at which the curves cross each other are important because they indicate when valves open and close

Point A – both left atrium and left ventricle are relaxed

  • Pressure sits at roughly 0 kPa

Between points A and B – atrial systole

  • Left atria contracts and empties blood into the left ventricle

Point B – beginning of the ventricular systole

  • Left ventricular pressure increases
  • AV valve shuts
  • Pressure in the left atria drops as the left atrium expands

Point C – pressure in the left ventricle exceeds that in the aorta

  • Aortic valve opens
  • Blood enters the aorta

Point D – diastole

  • Left ventricle has been emptied of blood
  • Muscles in the walls of the left ventricle relax and pressure falls below that in aorta
  • Aortic valve closes
  • AV valve opens

Point E – expansion of the left ventricle

  • There is a short period of time during which the left ventricle expands
  • This increases the internal volume of the left ventricle which decreases the pressure

Cardiac Cycle Labelled, downloadable AS & A Level Biology revision notes

Image showing the pressure changes within the aorta, left atrium and left ventricle during the cardiac cycle.

Exam Tip

The maximum pressure in the ventricles is substantially higher than in the atria. This is because there is much more muscle in the thick walls of the ventricles which can exert more force when they contract.


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