The right atrium, one of the two upper chambers of the heart, receives deoxygenated blood from the superior and inferior vena cavae and the coronary sinus, and pumps it into the right ventricle through the tricuspid valve.
Structurally it is thin walled muscular sac separated from the RV by the tricuspid valve.
Functionally it acts as a conduit for blood from the systemic venous circulation and directing it to the RV. It receives venous blood from the SVC and IVC and early in the cycle the blood is transferred passively and later in the cycle there is active contraction that enables the topping up of the right ventricle via the tricuspid valve into the RV.
The right atrium has three basic parts: the appendage, the vestibule, and the venous component.
Viewed from outside the dominant portion is the anterolaterally located triangular appendage 2, which unlike that of the left is large and enjoys a wide junction with the venous component. This junction is marked by a fat filled groove externally termed the terminal groove or sulcus terminalis which internally corresponds to the terminal crest or crista terminalis 2.
The sinus node lies in this groove close to the superior cavoatrial junction 2.
The vestibule is represented by a smooth muscular rim surrounding the tricuspid orifice.
The venous component as the name implies receives the caval veins and coronary sinuses. It is also smooth.
Viewed from within one can see the following structures:
Eustachian valve 2 : Fibromuscular tissue that inserts medially into the sinus septum and serves to guard the entrance of the IVC. In some cases it may be large and pose an obstacle to catheter passed from the IVC. The free border of the Eustachian valve forms the tendon of Todaro, one of the borders of the triangle of Koch that contains the AV node2.
Triangle of Koch 2 is demarcated by the tendon of Todaro posteriorly, the tricuspid valve anteriorly, the coronary sinus inferiorly and the central fibrous body at the apex. The central fibrous body is the land mark for penetration of the bundle of His 2.
Thebesian Valve : A crescentic flap that guards the orifice of the coronary sinus.
Size of the Right Atrium
The linear measurements for the normal right atrium are a short axis of 3.5 cms and a long axis of 5.5cms. Since it has the same volume as the left atrium it should be about the same size but comparison is not that easy for a few reasons, including the different orientations of the chambers as well as the fact that they often lie in different planes making comparison at a single level difficult.. The long axis of the right atrium is mostly in an A-P direction,whereas the long axis of the left atrium is mostly in a transverse direction. They also lie only a slightly different axial plane so theat theright strium (like most right sided structures
Normally, right and left atria are the same size.
Normal Right Atrium and Left Atrium
This axial image of the heart is through the mitral (right ) and tricuspid valve (left) right atrium and left atrium (right) which are normal and about the same size. The left ventricle with the papillary muscles and the right ventricle with its papillary muscle are well seen. Note the right sided structures tend to be anterior and left sided structures tend to be posterior. Note also that the right atrium and the left atrium are about the same size and shape in this view with flat walls.
Courtesy Ashley Davidoff MD copyright 2009 all rights reserved 27531d01.8s
Normal and Enlarged
These two images are enhanced CT images through the tricuspid valve. The first image is normal, while the second image demonstrates a TV that lies too far forward and low associated with a huge RA and a diminutive RV. This appearance is classical of a congenital condition called Ebstein’s anomaly. In this disease, the posterior leaflet is stuck down to the posterior wall of the RV and the anterior leaflet is larger than normal often giving a flapping sound to the listening ear. The anterior leaflet behaves like a flapping sail in the wind of the blood flow.
In the overlays, the RA is in royal blue and the RV is in light purple overlay. The normal TV in the first image is in pink while the malformed valve in the second image is in green. Note how large the RA is and how small the RV is in Ebstein’s anomaly.
Courtesy of Ashley Davidoff M.D. 32102 32101 copyright 2009
Clinical Considerations :
The right atrium is the site for catheter ablation of right atrial arrhythmias, AVNRT, for guiding transseptal puncture, for coronary sinus cannulation, and for correct positioning of atrial pacing leads or atrial septal occluder devices. For the interventionist, it is the most commonly used cardiac chamber for entering the heart 3.
After the “dance of the scaffold”, we positioned it rightward and inferior to the LA – with the added piece that it forms a border with the middle lobe of the right lung. As mentioned before, the right and left atria are structurally different beasts – as we say – as different as “chalk and cheese”.
Pectinate muscles, tenia saginata, and limbic bands, all make the right atrium an extremely interesting chamber, as opposed to the more boring smooth walled left atrium. Even as you look on the outside, the atrial appendages are different. The right appendage is more triangular in shape – like Snoopy’s nose and the left appendage is shaped like a crooked index finger, or some say like the tip of the map of South America.
Diseases of the RA, in isolation are rare but usually manifest as RA dilation secondary to RV dysfunction, tricuspid incompetence, pulmonic stenosis, intracardiac shunts or congenital disorders like Ebstein’s anomaly.
Diagnosis of RA disease is rarely suspected clinically but is an accompaniment of RV dysfunction which presents with signs of elevated jugular veins, hepatic congestion, hepatomegaly, ascites , bilateral pitting pedal edema. On examination a RV heave may be palpated in the left parasternal area. Chest radiography shows RV enlargement. EKG characteristics of RA enlargement are narrow P wave of increased amplitude (P pulmonale) on lead I1. Two dimensional echocardiography serves to accurately predict the size and function of the RA.