3.2.7 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

The events of the cardiac cycle allow the heart to pump blood to the lungs and body

Valves during the cardiac cycle table

Image showing the pressure changes within the aorta, left atrium and left ventricle during the cardiac cycle. This graph can be analysed step-by-step.

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

• Pressure sits at roughly 0 kPa

• Left atria contracts and empties blood into the left ventricle

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

• 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

• 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

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