The Cardiac Cycle: Skills (HL) (HL IB Biology)

Control of the cardiac cycle

• Control of the basic heartbeat is myogenic, which means the heart will beat without any external stimulus from the nervous system
  • This intrinsic rhythm means the heart beats at around 60 times per minute
• The sinoatrial node (SAN) is a group of cells in the wall of the right atrium; it controls the cardiac cycle as follows:
  • The SAN initiates a wave of depolarisation that spreads across the atria and causes the atria to contract
  • A region of non-conducting tissue prevents the depolarisation spreading straight to the ventricles
    • This delays the wave of depolarisation for long enough to allow the atria to finish contracting
  • The depolarisation is instead carried to the atrioventricular node (AVN); this is a region of conducting tissue between atria and ventricles
  • Conducting fibres carry the wave of excitation down the septum and out around the walls of the ventricles
    • The bundle of His is a collection of conducting tissue located in the septum
    • The Purkyne fibres spread around the ventricles
  • The ventricles depolarise from the base upwards, and the ventricles contract

Stages in the cardiac cycle table

Stage in sequence	Event
1	Sinoatrial node sends out a wave of excitation
2	Atria contract
3	Atrioventricular node sends out a wave of excitation
4	Purkyne tissue conducts the wave of excitation
5	Ventricles contract

Control of cardiac cycle diagram

The wave of depolarisation spreads across the heart in a coordinated manner

The events of 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
• The events of the cardiac cycle occur simultaneously in the left and right sides of the heart

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 heartbeat is initiated by the sinoatrial node
• 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

Cardiac cycle diagram

Valves during the cardiac cycle table

Stage in cardiac cycle	Atrioventricular valves	Semilunar valves
Atrial systole	Open	Closed
Ventricular systole	Closed	Open
Diastole	Open	Closed

Pressure changes during the cardiac cycle graph

The pressure changes during the cardiac cycle can be used to determine the events taking place

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