Back to last page  Atrial Fibrillation  What Is It?   Atrial
fibrillation affects the upper two chambers of the heart, the
atria. All blood circulates through both atria.  The heart is a
muscle. The walls of the chambers of the heart are made of
muscle cells.  Normally, the muscular walls of the atria
contract at the same time, pumping blood into the lower two
chambers (the ventricles). Then the walls of the ventricles
contract at the same time, pumping blood to the rest of the
body.  For the heart to work efficiently, the atria need to pump
first, and then the ventricles need to pump. How is this
coordinated? Normally, each heartbeat starts with an electrical
impulse that comes from a small part of the atrium called the
sinus node. That signal first causes the atria to beat, pumping
blood into the ventricles. Then the signal travels to another
part of the heart called the atrioventricular node. From there,
the signal travels down to the ventricles, and causes them to
beat, sending blood throughout the body.