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Cardiac cycle

  • The cardiac cycle is a series of events that occur in the heart during one complete heartbeat.

  • It consists of two main phases: diastole (relaxation) and systole (contraction).

  • During the cardiac cycle, the atria and ventricles contract and relax in a coordinated manner to pump blood throughout the body.

  • The duration of a single cardiac cycle depends on the heart rate, but at a normal resting heart rate, it takes about 0.8 seconds.

Cardiac cycle
Cardiac cycle

Phases of the Cardiac Cycle:

1. Atrial Diastole (Relaxation)

  • Both atria relax, allowing blood to flow into the atria from the pulmonary veins (left atrium) and vena cavae (right atrium).

  • The atrioventricular (AV) valves (tricuspid and mitral) are open, and blood passively flows into the ventricles.

2. Atrial Systole (Contraction)

  • The SA node triggers atrial contraction, pushing additional blood into the ventricles, completing ventricular filling.

3. Ventricular Diastole (Early)

  • The ventricles are relaxed and continue filling with blood from the atria. The AV valves remain open.

4. Ventricular Systole (Isovolumetric Contraction)

  • The ventricles contract as the electrical impulse passes through the AV node, Bundle of His, and Purkinje fibers.

  • Pressure builds, closing the AV valves (producing the first heart sound, "lub"), but the semilunar valves (aortic and pulmonary) remain closed, so no blood is ejected yet.

5. Ventricular Systole (Ventricular Ejection)

  • Ventricular pressure exceeds that of the aorta and pulmonary artery, forcing the semilunar valves open.

  • Blood is pumped from the left ventricle into the aorta and from the right ventricle into the pulmonary artery.

6. Ventricular Diastole (Isovolumetric Relaxation)

  • After ejection, the ventricles relax, and pressure drops, causing the semilunar valves to close (producing the second heart sound, "dub").

  • The AV valves remain closed during this phase.

7. Ventricular Diastole (Late)

  • As ventricular pressure drops further, it falls below atrial pressure, causing the AV valves to open, allowing blood to flow into the ventricles and initiating the next cycle.


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