Cardiac Cycle

Cardiac Cycle

Cardiac cycle is the term referring to all or any of the events related to the flow or pressure of blood that occurs from the beginning of one heart beat to the beginning of the next heart beat.
• The contraction phase is called as systole while the relaxation phase is called as diastole. Cardiac cycle is discussed below in the diagram.
• The frequency of cardiac cycle is the heart rate.

• Two sounds are heard normally through a stethoscope during each cardiac cycle. The first is a low, slightly prolonged “lubb” (first sound), caused by sudden closure of the mitral and the tricuspid valves at the
• start of ventricular systole. The second is a shorter, high pitched “dup” (second sound) caused by vibrations associated with closure of the aortic and pulmonary valves just after the end of the ventricular systole.

• The first sound has a duration of 0.15 second and a frequency of 25-45 Hz. The second sound lasts about 0.12 second with a frequency of 50 Hz. There is a gap or pause between the second sound and the next cardiac cycle. Each cardiac cycle takes 0.8 second to occur.

Table : Stages of cardiac cycle Phase AV Valves Semilunar valves Status of ventricles and atria
1 Atrial systole Open Closed • Atria contract and pump blood
• Ventricles, already partially filled from phase 5, receive last -30% of blood, for a final resting volume of approximately 130 mL.
2 Isovolumetric
Closed Closed • Ventricles begin to contract. Ventricular muscle initially shortens only a little, but intraventricular pressure rises sharply
• Ventricular volume unchanged
3 Ventricular
Closed Open • Pressures in left and right ventricle exceed pressures in aorta (80 mmHg) and pulmonary artery (10mmHg). Ejection is rapid at first, slowing down as systole progresses.
• Amount ejected each ventricle per stroke at rest is 70 – 90 mL. Approximately 50 mL of blood remains in each ventricle at the end of systole.
4 Isovolumetetric
Closed Closed • Valves closed as ventricles relax and pressure within ventricles drop below 120mmHg. This ends once ventricular pressure falls below Atrial pressure and AV valves open.
5 Ventricular
Open Closed • Ventricles relaxed
• Ventricles passively fill with approximately 70% of their final volume. As the ventricles fill, rate of filling decreases and the AV valves drift towards closing atria expand and are filling


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