Your heart has a pacemaker in the top, and when that pacemaker discharges, other cells, starting
in the top of your
heart and working to
the bottom, contract and forces
the blood out of
the heart. These cells that can respond to the slight electric current from the heart's pacemaker can, for some reason, cause slight discharges on their own. When that happens, the heart contracts or tries to before it should of.