Webster's New World Medical Dictionary

ATP
1
Acute thrombocytopenic purpura.
2
Adenosine triphosphate.
atresia
Absence of a normal opening, or failure
of a structure to be tubular. Atresia can affect many
structures in the body. For example, esophageal
atresia is a birth defect in which part of the esopha-
gus is not hollow, and with anal atresia, there is no
hole at the bottom end of the intestine.
atria
The plural of atrium.
atrial arrhythmia
See 
arrhythmia, atrial.
atrial fibrillation
See 
fibrillation, atrial.
atrial septal defect
A hole in the wall (septum)
between the upper chambers of the heart (atria).
Abbreviated ASD. ASD is a major class of heart mal-
formation. Usually, when clots in veins break off
(embolize), they travel first to the right side of the
heart and then to the lungs, where they lodge. When
there is an ASD, however, a clot can cross from the
right to the left side of the heart, and then pass into
the arteries as a paradoxical embolism. Once a clot
is in the arterial circulation, it can travel to the
brain, block a vessel there, and cause a stroke.
ASDs are surgically closed.
atrial septum
The wall between the right and left
atria of the heart.
atrioventricular
Pertaining to the upper cham-
bers of the heart (atria) and the lower chambers of
the heart (ventricles).
atrioventricular node
The electrical relay sta-
tion between the upper and lower chambers of the
heart. Abbreviated AV node. Electrical signals from
the atria must pass through the AV node to reach the
ventricles. The AV node, which controls the heart
rate, is one of the major elements in the cardiac
conduction system. The AV node serves as an elec-
trical relay station, slowing the electrical current
sent by the sinoatrial (SA) node before the signal is
permitted to pass down through to the ventricles.
This delay ensures that the atria have an opportunity
to fully contract before the ventricles are stimulated.
After passing the AV node, the electrical current
travels to the ventricles, along special fibers embed-
ded in the walls of the lower part of the heart.