What prevents the atria and the ventricles from contracting at the same time?
1. Pacemaker cells located in the atria fire before the pacemaker cells in the ventricles.
2. It takes time for epinephrine to diffuse from the atria to the ventricles to trigger contraction
3. The electrical signal generated in the right atrium is delayed at the AV node before passing to the ventricles.
4. The Na+ channels responsible for initiating ventricular contraction are inactivated and need to return to activated configuration to the electrically stimulated.