The events that occur at the beginning of the heartbeat and last until the beginning of next beat is called cardiac cycle. It lasts for 0.8 seconds. The series of events that takes place in a cardiac cycle.
PHASE 1:
Ventricular diastole- The pressure in the auricles increases than that of the ventricular pressure. AV valves are open while the semilunar valves are closed. Blood flows from the auricles into the ventricles passively.
PHASE 2:
Atrial systole – The atria contract while the ventricles are still relaxed. The contraction of the auricles pushes the maximum volume of blood to the ventricles until they reach the end-diastolic volume (EDV). EDV is related to the length of the cardiac muscle fibre. The more the muscle is stretched, the greater the EDV and the stroke volume.
PHASE 3:
Ventricular systole (isovolumetric contraction) – The ventricular contraction forces the AV valves to close and increases the pressure inside the ventricles. The blood is then pumped from the ventricles into the aorta without change in the size of the muscle fibre length and ventricular chamber volume (isovolumetric contraction).
PHASE 4:
Ventricular systole (ventricular ejection) – Increased ventricular pressure forces the semilunar valves to open and blood is ejected out of the ventricles without backflow of blood. This point is the end of the systolic volume (ESV).
PHASE 5:
(Ventricular diastole) – The ventricles begin to relax, pressure in the arteries exceeds ventricular pressure, resulting in the closure of the semilunar valves. The heart returns to phase 1 of the cardiac cycle.