Cardiac Cycle Notes

Cardiac Cycle

The cardiac cycle is the sequence of events that occur and repeat with every heartbeat. It consists of two major phases: systole and diastole. Systole refers to ventricular contraction, while diastole refers to ventricular relaxation.

Key Principles

Blood Flow: Blood always flows from areas of higher pressure to areas of lower pressure.
Contraction: Contraction increases pressure within a chamber.
Relaxation: Relaxation lowers pressure within a chamber.
AV Valves: Open when atrial pressures are higher than ventricular pressures, and close when the pressure gradient is reversed.
Semilunar Valves: Open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true.

Phases of the Cardiac Cycle

1. Atrial Depolarization

The cycle begins with the firing of the SA node, which stimulates the atria to depolarize.
This depolarization is represented by the P-wave on the ECG.

2. Atrial Contraction

Atrial contraction starts shortly after the P-wave begins.
This contraction increases the pressure within the atria, forcing blood into the ventricles.
Atrial contraction only accounts for a fraction of ventricular filling because the ventricles are already almost full due to passive blood flow.

3. AV Valve Closure (S1)

As atrial contraction completes, atrial pressure begins to fall, reversing the pressure gradient across the AV valves.
This causes the AV valves to close, producing the first heart sound, S1.
S1 marks the beginning of systole.
Ventricular depolarization, represented by the QRS complex on the ECG, is halfway through at this point.

4. Isovolumetric Ventricular Contraction

The ventricles begin to contract, rapidly building up pressure.
The semilunar valves (aortic and pulmonic) remain closed.
This phase is called isovolumetric contraction because no blood is ejected, and ventricular volume remains unchanged.

5. Ventricular Ejection

Ventricular ejection starts when ventricular pressures exceed the pressures within the aorta and pulmonary artery.
The aortic and pulmonic valves open, and blood is ejected out of the ventricles.
This is the rapid ejection phase.

6. Semilunar Valve Closure (S2)

As ventricular repolarization, reflected by the T-wave on the ECG, begins, ventricular pressure starts to fall and the force of ejection is reduced.
When ventricular pressures drop below aortic and pulmonary pressures, the semilunar valves close.
Closure of the semilunar valves produces the second heart sound, S2.
This marks the end of systole and the beginning of diastole.

7. Isovolumetric Ventricular Relaxation

The first part of diastole is isovolumetric.
The ventricles relax with all valves closed.
Ventricular pressure drops rapidly, but ventricular volume remains unchanged.
Meanwhile, the atria are being filled with blood, and atrial pressures rise slowly.

8. Ventricular Filling

Ventricular filling starts when ventricular pressures drop below atrial pressures.
This causes the AV valves to open, allowing blood to flow passively down the ventricles.