3.5.4 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
• 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

Valves during the cardiac cycle table

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

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 

So there is 1 cycle in 0.7 seconds

Step 2: Calculate how many heart beats occur per second

Divide by 0.7 to find out how many cycles in 1 second

1  0.7 = 1.43 beats in 1 second

Step 3: Calculate how many heart beats occur per minute

Multiply by 60 to find out how many cycles in 60 seconds

1.43  60 = 85.71 beats in 60 seconds

So the heart rate is 85.71 beats / min