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.

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