The cardiac cycle can be divided into 2 major events: systole and diastole, both of which sub-divide into smaller phases. Systole refers to the contraction of the heart muscle, whilst diastole refers to the relaxation of the heart muscle. Both are equally important for the normal functioning of the heart, as diastole allows filling with blood whilst systole causes the pumping out of the blood. It is important to note that:
- blood flows from higher to lower pressure
- contraction increases the pressure within the chamber while relaxation lowers the pressure
- valves open/close according to pressure gradients
Phases of the cardiac cycle
- Atrial depolarization/contraction
- Isovolumetric Contraction
- Rapid Ejection
- Reduced Ejection
- Isovolumetric Relaxation
- Ventricular Filling
Atrial systole starts after the P wave of the ECG and lasts 0.1 seconds, which is then followed by atrial diastole that lasts 0.7 seconds.
Ventricular systole starts close to the end of the R wave and ends just after the T wave, lasting for about 0.3 seconds. It is then followed by ventricular diastole that lasts 0.5 seconds.
Heart Sounds
Heart sounds are caused primarily from the turbulence in the blood flow created by the closure of the valves. While there are 4 heart sounds per cardiac cycle, only the 1st and 2nd heart sounds are loud enough to be auscultated.
S1 HEART SOUND is a long booming sound caused by the closure of the atrioventricular valves soon after ventricular sistole begins.
S2 HEART SOUND is a short sharp sound caused by the closure of the semilunar valves towards the end of the ventricular systole.
The 3rd heart sound happens due to the blood turbulence during rapid ventricular filling, while the 4th heart sound happens due to blood turbulence during the atrial systole, both of which are not long enough to be auscultated.
Below you can find a collection of videos that can help provide a more visual approach to the cardiac cycle.
The Cardiac Cycle
Heart Sounds
Special thanks to the creators of the featured videos on this post, specifically Youtube Channels Registered Nurse RN and Alila Medical Media.
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Claire is keen about public education on health-related subjects as well as holistic patient-centered care. She is also passionate about spreading awareness on the negative effects that domestic abuse leaves on its victims’ mental, emotional, social and physical wellbeing.
Claire aspires to continue studying following completion of her Nursing Degree, because she truly believes in lifelong education.
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