Pressroom Atrial fibrillation is an abnormality of heart rhythm in which chambers of the heart no longer contract in an organized manner. Heart rate often becomes irregular and may be very fast, producing palpitations. The heart has a right side and a left side. Atria are relatively thin-walled chambers, whereas the ventricles are much more muscular.
Blood passes from the atria into the ventricles through two processes.
During the resting phase, when the ventricles are not contracting, the tricuspid and mitral valves open. Some of the blood that has accumulated in the atria passively flows through the tricuspid and mitral valves into the right and left ventricles, respectively. The atria then contract, pumping blood out and into the ventricles. Once the ventricles fill with blood, they contract, pumping blood out of the ventricles, into the lungs, and to the body. Contractions of the different chambers of the heart are normally organized in a specific manner.