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Cardiac Cycle

The cardiac cycle is the complete sequence of pressure and volume changes that occur in the heart from the beginning of one heartbeat to the beginning of the next. It is the continuous pattern of systole (contraction) and diastole (relaxation) that ensures the heart acts as an efficient double pump. The movement of blood is strictly controlled by pressure gradients and the opening/closing of the heart's valves.

The cycle is typically described in three main sequential stages:

  1. Atrial and Ventricular Diastole (Relaxation and Filling): The atria and ventricles are relaxed. Blood passively flows into the atria from the vena cava and pulmonary vein, and from the atria into the ventricles (since the atrioventricular valves are open).
  2. Atrial Systole (Atrial Contraction): The atria contract, which increases pressure in the chambers, forcing the remaining blood from the atria into the relaxed ventricles.
  3. Ventricular Systole (Ventricular Contraction and Ejection): The ventricles contract, dramatically increasing ventricular pressure. This closes the atrioventricular valves (producing the first heart sound) and then forces the semilunar valves open, ejecting blood into the aorta and pulmonary artery.