The document provides a detailed anatomical description of the interior of the right atrium of the heart, including its division into atrium proper and sinus venarum, and notable structures such as the crista terminalis, fossa ovalis, and various openings. It outlines embryological aspects, abnormalities like Ebstein anomaly and atrial septal defects, and applied anatomy regarding the right auricular appendage and the triangle of Koch, highlighting their clinical significance. Additionally, it touches on radiological techniques for assessing heart chamber contours and dimensions.

1. Interior of right atrium
BY
DR.P.SASIDHAR
1ST YEAR PG
SIDDHARTHA MEDICAL COLLEGE
VIJAYWADA

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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8.  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.

9. 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.

10.  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.

13. Anterior cusp
Septal cusp
Posterior cusp

14. Embryological importance
 The interior of the right atrium has three
anatomically distinct regions, each a remnant
of embryological development.
1.The posterior portion of the right atrium termed sinus
venarum.
2.Anterior segment, lined by horizontal, parallel ridges of
muscle
bundles, 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
septum
secundum

15. 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.

16.  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.

17. Applied aspect in relation to
embryology
Ebstein anamoly- tricuspid valve is displaced
towards the apex of the right ventricle, anterior
leaflet of the valve enlarged, results in
hypertrophy of right atrium with a small right
ventricle.
Premature closure of the oval foramen- leads to
massive hypertrophy of the right atrium and
ventricle and underdevelopment of left side of the
heart, results in death shortly after birth.
About 1/3 rd of the individuals have a slit like opening
at the fossa ovalis-but it is physiologically closed.

18. 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.

22. Applied anatomy
Right 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.

23. Radiological aspect
Through 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.