Cardiac Cycle

This lesson covers: 

1. The stages of the cardiac cycle

Stages of the cardiac cycle

The cardiac cycle is the sequence of contraction and relaxation of the cardiac muscle in the walls of the heart. It has three main stages: atrial systole, ventricular systole, and diastole.

Stage 1 - Atrial systole

• The ventricles relax, and the atria contract.
• This increases the atrial pressure.
• The atrioventricular valves open.
• Blood flows into the ventricles.

Stage 2 - Ventricular systole

• The ventricles contract, and the atria relax.
• The ventricular pressure increases.
• The semi-lunar valves open, and the atrioventricular valves close.
• Blood flows into the arteries.

Stage 3 - Diastole

• The ventricles and atria relax.
• The semi-lunar valves close.
• Blood flows passively into the atria.

Cardiac output

Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute.

We can calculate it as follows:

1. Measure the heart rate - This is the number of heart beats per minute.
2. Measure the stroke volume - This is the volume of blood that is pumped out of the left ventricle during ventricular systole.
3. Calculate the cardiac output - Cardiac output = heart rate× stroke volume