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.
Systole and diastole, when not specified otherwise, refer to ventricular contraction and relaxation,
respectively.
Reminders: - 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 open when ventricular pressures are higher than aortic/pulmonary
pressures, and close when the reverse is true.
The cycle is initiated with the firing of the SA node that stimulates the atria to depolarize.
This is represented by the P-wave on the ECG.
Atrial contraction starts shortly after the P-wave begins, and causes the pressure within
the atria to increase, forcing blood into the ventricles.
Atrial contraction, however, only accounts for a fraction of ventricular filling, because
at this point, the ventricles are already almost full due to passive blood flow down
the ventricles through the open AV valves.
As atrial contraction completes, atrial pressure begins to fall, reversing the pressure gradient
across the AV valves, causing them to close.
The closing of the AV valves produces the first heart sound, S1, and marks the beginning
of systole.
At this point, ventricular DE-polarization, represented by the QRS complex, is half way
through, and the ventricles start to contract, rapidly building up pressures inside the ventricles.
For a moment, however, the semilunar valves remain closed, and the ventricles contract
within a closed space.
This phase is referred to as isovolumetric contraction, because no blood is ejected and
ventricular volume is unchanged.
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.
As ventricular repolarization, reflected by the T-wave, 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, marking the end of systole and beginning of diastole.
Closure of semilunar valves produces the second heart sound, S2.
The first part of diastole is, again, isovolumetric, as the ventricles relax with all valves closed.
Ventricular pressure drops rapidly but their volumes remain unchanged.
Meanwhile, the atria are being filled with blood and atrial pressures rise slowly.
Ventricular filling starts when ventricular pressures drop below atrial pressures, causing
the AV valve to open, allowing blood to flow down the ventricles passively.
The atria contract to finish the filling phase and the cycle repeats itself.