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.