The heart has four areas, or chambers. During each heartbeat, the two upper chambers (atria) contract, followed by the two lower chambers (ventricles). This is directed by the heart's electrical system.
The electrical impulse begins in an area called the sinus node, located in the right atrium. When the sinus node fires, an impulse of electrical activity spreads through the right and left atria, causing them to contract, forcing blood into the ventricles.
Then the electrical impulses travel in...
The quivering upsets the normal rhythm between the atria and the
lower parts of the heart, the ventricles. And the ventricles may beat fast and without a regular rhythm.
This is dangerous
because if the heartbeat isn't strong and steady, blood can
collect, or pool, in the atria. And pooled blood is more likely to form clots. Clots can travel to the brain, block blood flow, and cause a stroke.
Interactive Tool: What Is Your Risk for a Stroke if You Have Atrial Fibrillation?