Alila Academy

The cardiac cycle refers to the sequence of events that occur and repeat with every heartbeat. 

It can be divided into 2 major phases: systole and diastole, each of which subdivides into several smaller phases (1 to 6 on diagram). A typical cycle lasts for about 0.8 second.

Systole = Contraction; There are 2 events with systole: 

- Atrial Systole = Atrial Contraction; 

- Ventricular Systole = Ventricular Contraction

Diastole = Relaxation; There are 2 events with diastole: 

- Atrial Diastole = Atrial Relaxation; 

- Ventricular Diastole = Ventricular Relaxation

However, systole and diastole, when not specified otherwise, refer to ventricular contraction and relaxation, respectively.

On image (Wigger's diagram): SYSTOLE (ventricular) = phases 2, 3, 4

DIASTOLE (ventricular) = phases 1, 5, 6

Basic principles of flow and pressure:

- Blood flows from higher to lower pressure. 

- Contraction increases the pressure within a chamber, while relaxation lowers the pressure.

- AV valves open when atrial pressures are higher than ventricular pressures and close when the pressure gradient is reversed. Similarly, semilunar valves (aortic and pulmonary valves) open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true.

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