3.2.7- The Cardiac Cycle

Question 1

What is the cardiac cycle?

Answer 1

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

Question 2

Describe how different volume and pressure changes allow valves to open and close and different heart muscles to contract and relax.

Answer 2

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

Question 3

Describe atrial systole.

Answer 3

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

Question 4

Describe ventricular systole.

Answer 4

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

Question 5

Describe diastole.

Answer 5

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