The Cardiac Cycle (Cambridge (CIE) A Level Biology): Revision Note

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

• One cardiac cycle is followed by another in a continuous process

  • There is no gap between cycles where blood stops flowing

Volume and pressure changes

• 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

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

  • This forces the AV valves to close, preventing back flow of blood

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

  • This forces the semilunar (SL) valves 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 of atrial systole

• The cycle then begins again with atrial systole

• Throughout the cardiac cycle, heart valves open and close as a result of pressure changes 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

• Valves are an important mechanism to stop blood flowing backwards

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

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