Cardiac Cycle

The cardiac cycle is the complete sequence of events from the start of one heartbeat to the start of the next, and it repeats roughly every 0.8 seconds. It is conventionally broken into five phases. In Phase 1 (ventricular diastole), atrial pressure exceeds ventricular pressure, so the AV valves are open (while the semilunar valves stay shut) and blood flows passively from the atria into the relaxed ventricles. In Phase 2 (atrial systole), the atria actively contract while the ventricles remain relaxed, pushing the last, largest volume of blood into the ventricles until they reach their maximum filling, the end diastolic volume (EDV) - the more the cardiac muscle fibres are stretched at this point, the greater both the EDV and the eventual stroke volume will be. In Phase 3 (ventricular systole - isovolumetric contraction), the ventricles begin contracting, which forces the AV valves shut and sharply raises pressure inside the ventricles, but because all valves are momentarily closed no blood can yet leave, so chamber volume and muscle-fibre length stay constant even as pressure builds. In Phase 4 (ventricular systole - ventricular ejection), ventricular pressure finally exceeds the pressure in the great arteries, forcing the semilunar valves open so blood is ejected out without any backflow, leaving behind the end systolic volume (ESV) - the blood that remains in the vent …