The Cardiac Cycle

What is the cardiac cycle?

The sequence of events which makes up one heartbeat.

Systole means…

Contracting.

Diastole means…

Relaxing.

Movement of blood is from a _______ to ________ pressure?

High to low pressure.

Conventionally, blood flow is described as starting with atrial systole. What is atrial systole?

When the atria contracts.

Summarise what happens in atrial systole.

• Atria contract. This increases the pressure in the atria.

• The atrio-ventricular valves open, allowing blood to flow through into the ventricles.

• At this point, the ventricles are in diastole (relaxed).

• The pressure developed during this contraction is not very great due to the thin atrial walls.

• Backflow is prevented by the valves closing.

Summarise what happens in ventricular systole.

• When the ventricles are full, they contract from the apex upwards.

• The pressure in the ventricles increases rapidly and quickly exceeds that of the atria, pushing the atrioventricular valves shut.

• Semi-lunar valves in the aorta and pulmonary artery open.

• Blood is pushed upwards and into the arteries.

Summarise what happens in ventricular diastole.

• The ventricles relax, causing the pressure to drop.

• The semi-lunar valves snap shut to prevent backflow of blood from the arteries (into the ventricles).

Summarise what happens in diastole.

• The whole of the heart muscle relaxes during diastole.

• Blood from the veins flows into the atria.

• The cardiac cycle begins again.

Describe the pressure changes in the heart during time interval A.

• The atria are contracting, increasing the atrial pressure.

• The ventricles are relaxed and the pressure in the ventricles is lower than the atrial pressure.

• The slight increase in pressure in the ventricles is because blood is being pushed into the ventricles.
  

Describe the pressure changes in the heart during time interval B.

• The pressure in the ventricles increases as the ventricles contract.

• The pressure in the ventricles rises above that of the atria at 0.2 seconds and the atrioventricular valves (tricuspid and bicuspid) close.

• At 0.24 seconds the pressure in the ventricles rises sharply and the semi-lunar valves open.

Describe the pressure changes in the heart during time interval C.

• The ventricular systole causes blood to enter the arteries (pulmonary artery and aorta) and the pressure in the arteries rises.

• As the ventricles start to relax, the pressure falls until, at 0.56 seconds, the pressure in the ventricles falls below that in the arteries, the semi-lunar valves fill with blood and close.

Describe the pressure changes in the heart during time interval D.

• The ventricles continue to relax, and all heart valves are closed.

• At 0.6 seconds, the pressure in the ventricles falls below that of the atria and the atrioventricular valves open.

Describe the pressure changes in the heart during time interval E.

• The heart is in diastole, the atria and ventricles are relaxed and blood flows from the veins, through the atria and into the ventricles, causing a slight increase in pressure as the ventricles begin to fill.