The Cardiac Cycle
THE CARDIAC CYCLE
The cardiac cycle refers simply to the steps that are undertaken by the heart as it goes from filling, to pumping the blood systematically, to filling once again.
The phases of the cardiac cycle:
1. Isovolumetric Contraction
This is the point between the closure of the mitral valve and the opening of the aortic valve. The heart is contracted but valves are closed.
2. Systolic Ejection
The heart squeezes and blood is ejected through the aortic valve. This phase can be considered the phase between the time the aortic valve opens and closes.
3. Isovolumetric Relaxation
This is the period of time between the closure of the aortic valve and the opening of the mitral valve.
4. Rapid filling phase
After the opening of the mitral valve, blood pools rapidly into the left ventricle.
5. Slow filling phase
At this point, blood flows into the LV slowly as the mitral valve is about to close.
Important heart sounds that occur during the cardiac cycle:
S1 – this is the 1st heart sound and represents the closing of the tricuspid and mitral valves
S2 – this is the 2nd heart sound and represents the closing of the pulmonary and aortic valves
S3 – this is the 3rd heart sound and represents the end of the rapid ventricular filling (can be associated with congestive heart failure)
S4 – this is a heart sound heard only if there is a stiff ventricle (is associated with ventricular hypertrophy)
The following graph represents the phases of the cardiac cycle: