atrio-, atri- +

(Latin: entrance hall or chamber; upper heart chamber; central room)

atrial (adjective)
1. Relating to or belonging to the central area in a building that is open to the sky.
2. The chamber that is connected to other chambers or passageways; especially, one of the two upper chambers of the heart.
atrial electrogram, high right atrial electrogram (s); atrial electrograms, high right atrial electrograms (pl) (nouns)
An intracardiac electrogram which records electrical potentials within the upper region of the right upper chamber of the heart that receives blood from the veins and forces it into the lower part of the heart that moves blood to the rest of the body, obtained by the introduction of electrodes high in the upper part of the heart near the sinus node or the small mass of specialized cardiac muscle fibers located in the back wall of the right upper space of the heart: "The intracardiac electrogram is used in localizing conduction blocks (delays in the normal flow of electrical impulses that cause the heart to beat) and diagnosing arrhythmias (abnormal rates or rhythms) of the heart."

atrial fibrillation (s), atrial fibrillations (pl) (nouns)
Extremely rapid, incomplete contractions of the atria (chamber of the heart that receives blood from the body and transmits it to the ventricles) resulting in irregular intensities and rhythms and uncoordinated movements of the heart.
atriomegaly (s), atriomegalies (pl) (nouns)
An abnormal enlargement of a chamber or part of the heart which may be one of the two smaller chambers of the heart or both of them (atria).

The atria consist of open spaces with recessed walls. The right atrium (space) receives deoxygenated blood from the body through the vena cava and pumps it down into the right ventricle which then sends it to the lungs and the left atrium receives oxygenated blood from the lungs and pumps it down into the left ventricle which delivers it to the body.

atrioseptoplasty (s) (noun), atrioseptoplasties (pl)
Repair of an atrial septal defect (wall between the upper chamber of the heart) by means of plastic surgery.
atrioseptostomy (s) (noun), atrioseptostomies (pl)
Establishment of a communication between the two upper chambers of the heart.
atriotomy (s), atriotomies (pl) (nouns)
A surgical opening of a chamber or space of the heart.
atrioventricular (adjective)
Relating to both the atria (chambers) and the ventricles of the heart; especially, to a connecting conduction event or anatomic structure between the atria and ventricles (a chamber of the heart that receives blood from an atrium (heart chamber) and pumps it to the arteries): "The atrioventricular sections of the heart includes the upper chambers of he heart (atria) and the lower chamber of the heart (ventricles)."
atrioventricular node, AV node (s); atrioventricular nodes (pl) (nouns)
The electrical relay station between the upper and lower chambers of the heart: "Electrical signals from the atria (upper chambers) must pass through the atrioventricular node in order to reach the ventricles (two lower chambers of the heart)."

"The atrioventricular note, or AV node, which controls the heart rate, is one of the major elements in the cardiac conduction system (specialized tissue that carries bodily electrical impulses)."

atrium (s), atria (pl) (nouns)
1. A court; the central hall or court of a Roman house.
2. A covered court or portico in front of the principal doors of churches, etc.
3. In a modern house, a central hall or glassed-in court that may be used as a sitting-room, having rooms opening off it, sometimes at more than one level.
4. In a public building, usually a skylit central court rising through several stories and surrounded by galleries at each level with rooms (shops, offices, etc.) opening off them.
5. In physiology, either of the two upper cavities (left and right atrium) of the heart into which the veins pour the blood.
6. In zoology, a large cavity into which the intestine opens.
7. In medicine, a chamber or cavity to which are connected several chambers or passageways: "On both sides of the heart, the atrium is the chamber that leads to the ventricle which is one of the two lower parts of the heart that moves blood to the rest of the body."
interatrial (adjective)
Located or situated between the atria (chambers) of the heart.
sinoatrial; sinuatrial; sinoauricular (adjectives)
A reference to the sinus venosus (sinus of venae cavae referring to either of two large veins that return oxygen-depleted blood to the right chamber of the heart).