Interior of right atrium

BYDR.P.SASIDHAR

1ST YEAR PG SIDDHARTHA MEDICAL COLLEGE

VIJAYWADA

Interior of right atrium divided into 2 parts by thepresence of vertical groove on the outside along theright border of the heart termed sulcus terminalis,indentation of which is present on the inside as a

ridgetermed crista terminalis.1.Rough anterior part-termed atrium proper with

musculi pectinati.2.Smooth posterior part termed sinus venarum

which consists of openings of superior and inferior vena cava, coronary sinus, foramina venarum minimarum, intervenous tubercle.

And then the two septa limiting the right atrium– Interatrial septum-1.It is of varied thickness, usually paper thin at the

fossa ovalis.2.Consists of fossa ovalis and limubs fossa ovalis or

annulus ovalis which is the margin of the fossa.Right atrioventricular septum- containing an orifice

of the same name, guarded by a tricuspid valve.

Crista terminalis-1.A smooth muscle ridge extending from the

upper part of the atrial septum, passes laterally in front of the opening of superior vena cava and skirts rounds the right side of the opening.

2.It runs downwards along the right wall of atrium coinciding with the external sulcus terminalis.

Rough anterior part-1.Musculi pectinati arise at right angles from the

crista terminalis and pass downwards and forwards to the right atrial appendage or the right auricle as transverse muscular ridges like that of a comb.

2.In the auricle some of these muscle fibres show a reticular pattern.

Sinus venarum Opening of superior vena cava-1.Situated in the upper and posterior part of the atrium, directed

downwards and forwards.2.It possesses no valve. Opening of Inferior vena cava-1.Situated in the lower and posterior part of the atrium, close to

the atrial septum. It is guarded by a rudimentary semilunar shaped valve called eustachian valve which is formed by duplication of endocardium containing a few muscle fibres.

2.The valve is rarely is absent and sometimes appear cribriform.

3.The valve has right and left horns which are continous with the lower end of the crista terminalis and the anterior limb of limbus fossa ovalis respectively.

Opening of coronary sinus-1.Situated in between the opening of inferior vena cava

and the right atrioventricular orifice, in the lower part of atrial septum.

2.Thebesian valve is one that is guarding it and is rudimentary.

Foramina venarum minimarum-1.Minute openings of venae cordis minimi(Thebesian veins),

more numerous on the septal wall.2.Anterior cardiac veins and sometimes the right marginal vein

drain into the atrium through these channels. Intervenous tubercle-1.Small muscular conical projection immediately below the

opening of superior vena cava.2.It is scarcely visible in man, probably regulate the blood of the

superior vena cava.3.It is also known as Intervenous tubercle of Lower.

Atrial septum Fossa ovalis-Oval or saucer shaped shallow depression in the lower

part, floor of which is thin. Limbus fossa ovalis-1.Sickle shaped sharp prominent margin surrounding upper, anterior

and posterior margins of the fossa ovalis appearing like a rim.2.Its anterior limb is continuous with the left horn of valve of inferior

vena cava. 3.Sometimes a small slit like opening is present at the upper margin

of the fossa ascending beneath the rim and communicates with the left atrium.

Torus aorticus(Aortic mound)- It is an elevation in the anterosuperior part of the septum caused by the bulging of the right posterior aortic sinus of the ascending aorta.

Tendon of todaro- 1.A subendocardial ridge which is a continuation of the

eustachian valve of the inferior vena cava and the thebesian ring of the coronary sinus.

Triangle of Koch-1.Triangular area bounded in front by the base of the septal

leaflet of the tricuspid valve, behind by the anteromedial margin of the opening of the coronary sinus and above by the tendon of todaro.

2.Ventrocaudal to the this there is the membranous part of the atrioventricular septum.

Anterior cusp

Septal cusp

Posterior cusp

Embryological importance The interior of the right atrium has threeanatomically distinct regions, each a remnantof embryological development.1.The posterior portion of the right atrium termed sinus venarum. 2.Anterior segment, lined by horizontal, parallel ridges of musclebundles, named musculi pectinati.3.The atrial septum

The sinus venarum from the right horn of the sinus venosus. The pectinate muscles from the primitive atrium. Atrial septum from the embryonic septum primum and septumsecundum

Embryological development From the third week, after development of the primitive heart

tube, the primitive atria are separated from the sinus venousus by a segmentation termed sinoatrial ring.

At the end of fourth week, sickle shaped crest grows from the roof of the common atrium into the lumen, towards the endocardial cushions.

This crest is the first part of the septum primum. The opening between the lower rim of the septum primum and

the endocardial cushions is called as the ostium primum. With further development the superior and inferior endocardial

cushions grow along the edge of the septum primum, closing the ostium primum.

Even before the closure is complete, cell death causes perforations in the upper part of the septum primum which in due course will form the ostium secondum by coalescence of these perforations.

When the lumen of the right atrium expands during the further development a new cresent shaped fold appears, this new fold is called septum secundum.

Its anterior limb grows downwards but never completes the fusion leaving the opening called foramen of ovale.

Upper part of the septum primum disappears and rest of it forms the valve of the ostium secundum.

Applied aspect in relation to embryology

Ebstein anamoly- tricuspid valve is displacedtowards the apex of the right ventricle, anteriorleaflet of the valve enlarged, results inhypertrophy of right atrium with a small rightventricle.Premature closure of the oval foramen- leads tomassive hypertrophy of the right atrium andventricle and underdevelopment of left side of theheart, results in death shortly after birth.About 1/3 rd of the individuals have a slit like openingat the fossa ovalis-but it is physiologically closed.

Atrial septal defects(ASD)-multiple types are there, but the usually encountered one and the one with some clinical importance is the ostium secundum defect, characterised by a large opening between the right and left atria.

Cor triloculare biventriculare- serious form of ASD, also termed as common atrium is characterised by complete absence of the atrial septum.

Applied anatomyRight auricular appendage is a potential site for formation

of thrombi, can lead to pulmonary embolism. It is often excised in cardiopulmonary bypass.

Triangle of koch-anatomical landmark for atrioventricular (AV)node also known as Tawara’s node.It is just above the opening of coronary sinus, at the level of lower part of sulcus terminalis.

Crista terminalis and atrial septum are the sites of genesis of arrhythmias.

Inferior vena caval opening is used for sending catheters owing to its wider diameter.

Fossa ovalis is used for transseptal puncture.

Radiological aspectThrough X-ray we can get to know the contour of

the heart and can find abnormalities of the heart chambers and great vessels to some extent, most commonly posteroanterior(PA) and lateral view are employed.

Ultrasonography helps in measuring the various dimensions of heart chambers, commonly employed are 4-chamber apical view, subxiphoid view, parasternal long axis and parasternal short axis views.