Each
heartbeat is called a
cardiac cycle, consisting of three stages:
1) The two
atria are in
systole (contraction), while the two
ventricles are in
diastole (relaxation), resulting in
blood moving from the atria to the ventricles. This lasts for 0.15 seconds.
2) The two ventricles are now in systole, while the atria relax, resulting in blood moving to the
lungs, and into the
body. This lasts for 0.30 seconds.
3) Finally, both the atria and the ventricles are in diastole, a complete
rest. This lasts for 0.40 seconds.
So, the total time for one cardiac cycle is 0.85 seconds. It's these three stages which make up the
sound of the heartbeat: lub dub pause.
Control and coordination of the cardiac cycle is the job of
nodal tissue.