Cardiac Cycle Phases

The cardiac cycle consists of phases that coordinate the heart's contractions and relaxations, ensuring efficient blood flow. Understanding these phases is crucial in Anatomy and Physiology II, as they highlight how the heart maintains circulation throughout the body.

1. Atrial Systole

   • The atria contract, pushing blood into the ventricles.
   • Atrial pressure increases, causing the AV valves to open.
   • This phase accounts for approximately 20-30% of ventricular filling.

2. Ventricular Systole

   • The ventricles contract, increasing pressure and closing the AV valves.
   • Blood is ejected into the aorta and pulmonary artery.
   • This phase is divided into isovolumetric contraction and ejection phases.

3. Atrial Diastole

   • The atria relax and fill with blood from the veins.
   • AV valves remain closed during ventricular systole.
   • Atrial pressure gradually increases as blood fills the chambers.

4. Ventricular Diastole

   • The ventricles relax, allowing them to fill with blood.
   • The aortic and pulmonary valves close to prevent backflow.
   • This phase includes isovolumetric relaxation and rapid filling.

5. Isovolumetric Contraction

   • Occurs at the beginning of ventricular systole.
   • All heart valves are closed, and volume remains constant.
   • Ventricular pressure rises sharply as the myocardium contracts.

6. Rapid Ejection

   • Blood is rapidly ejected from the ventricles into the arteries.
   • Ventricular pressure exceeds arterial pressure, opening the semilunar valves.
   • This phase is characterized by a steep drop in ventricular volume.

7. Reduced Ejection

   • Blood ejection continues but at a slower rate.
   • Ventricular pressure begins to decrease as the contraction wanes.
   • This phase prepares the heart for the next diastolic filling.

8. Isovolumetric Relaxation

   • Occurs at the beginning of ventricular diastole.
   • All heart valves are closed, and volume remains constant.
   • Ventricular pressure drops rapidly as the myocardium relaxes.

9. Rapid Ventricular Filling

   • Blood rushes into the ventricles from the atria.
   • AV valves open due to the pressure gradient.
   • This phase accounts for the majority of ventricular filling.

10. Reduced Ventricular Filling

    • Blood flow into the ventricles slows down.
    • Atrial pressure continues to rise as the atria fill.
    • This phase prepares the heart for the next cycle of atrial systole.