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pulmonary artery, and there is a left ventricle, which pumps blood through the aortic valve to the aorta. The ventricles are separated by the interventricular septum.

Atrium - The upper chambers of the heart. The atria are separated from the ventricles via valves, and from each other by the atrial septum, which joins with the interventricular septum immediately after birth upon the closure of a flap (called the

foramen ovale) that allows fetal blood flow to differ from the required blood flow for breathing and oxygenation.

Valve - A one-way flap that prevents the backflow of blood. There are four valves in the heart: the aortic, the pulmonary (also known as semilunar), the tricuspid on the right side, and the mitral (also known as bicuspid) on the left side. Bi- and tri-cuspid refer to the number of tissue flaps that make the valve. The tissue flaps are held taunt by tendons called chordae tendineae, the muscles along the flaps are papillary muslces. Semilunar refers to the shape of the pulmonary valve. Upon ventricle contraction, the valves stay closed to ensure efficient blood flow forward.

Contraction - The mechanical function of the heart muscle is to pump blood, and it does so by contracting, or squeezing. Under normal conditions, the atria contract at the same time, called atrial systole, which pushes blood into the ventricles. The atria begin to relax as the ventricles contract, called ventricular systole, which pushes blood into the lungs or aorta, depending on the side of the heart it is in. The heart then relaxes, called diastole, which is followed by atrial systole to keep the cycle going. During diastole, blood flows into the atria. This two-step systole is part of the well-known "thump thump" of the heartbeat, with the noise actually corresponding to valve closures caused by the contractions.

Node - The involuntary electrical impulses that cause contraction travel through "nodes" in the cardiac tissue. The sinoatrial (SA) node is the natural pacemaker of the heart, initiating systole in the right atrium. The signal travels to the atrioventricular (AV) node in the atrial septum. Other parts of the cardiac electrical conductance system are Purkinje fibers and the bundle of His. The electrical conduction fibers in the heart muscle are modified myocytes with relatively low action potentials for electrical conduction.

Fibrillation - A type of arrhythmia, or an irregular heart rhythm, due to problems in the cardiac nodes, affecting contraction. Atrial fibrillation is estimated to affect more than 2 million Americans, and results in inadequate emptying of the atria. Ventricular fibrillation is more serious and a cause of sudden death.

Learn more about this author, Alicia M Prater PhD.
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