The Cardiac Cycle (CIE A Level Biology)

• 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
• 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

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

Stages of the cardiac cycle diagram

The cardiac cycle has three stages: atrial systole, ventricular systole, and diastole

Valves during the cardiac cycle table

Pressure changes during the cardiac cycle graph

The cardiac cycle graph can be analysed step by step

Analysing the cardiac cycle

• The lines on the graph represent the pressure of the left atrium, aorta, and the left ventricle
• The points at which the lines cross each other are important because they indicate when valves open and close

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

Between points A and 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

Between points D and 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