CARDIAC CYCLE
The cardiac cycle is a period from the beginning of one heart beat to the
beginning of the next one. It consists of two parts:
1. Ventricular contraction called systole.
2. Ventricular relaxation called diastole.
Each part of the cardiac cycle consists of several phases characterized by either
a strong pressure change with constant volume or a volume change with a relatively
small change in pressure.
Systole includes:
1. Isovolumic contraction.
2. Ejection.
Diastole includes:
3. Isovolumic relaxation.
4. Rapid ventricular filling.
5. Slow ventricular filling (diastasis).
6. Atrial contraction.
The duration of the cardiac cycle is inversely proportional to the heart rate. The
cardiac cycle duration increases with a decrease in the heart rate and on the other
hand it shortens with increasing heart rate. At a normal heart rate of 75 beats per
minute, one cardiac cycle lasts 0.8 second. Under resting conditions, systole
occupies ⅓ and diastole ⅔ of the cardiac cycle duration. At an increasing heart rate
(e.g. during an intensive muscle work), the duration of diastole decreases much more
then the duration of systole.
In the following six chapters the individual phases of the cardiac cycle are
described. The description contains:
¾ Mechanical events in the heart (the contraction on pictures is represented by a
colour change of the myocardium).
¾ Pressure and volume changes in both the atria and the ventricles. The pressure
changes in the right atrium are seen in the recording of the venous pulse.
¾ Pressure changes in the arteries – arterial pulse.
¾ Electrical activity of the heart – electrocardiogram (ECG)
¾ Heart sounds or phonocardiogram.