THE
RIGHT ATRIUM:
The right
atrium of the heart has three characteristic components. The venous
components is the most obvious in the normal heart.
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It has smooth wall and receives
the superior and inferior caval veins. The coronary sinus also drains
into this component.
The appendage
is the most constant part of the right atrium and is anatomically
distinct. It is triangular structure which adjoins the venous
component over a wide junction. The internal surface of the appendage
is corrugated by the parallel pectinate muscles, which originate from
the terminal crest.
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This crest
marks the junction with the venous part and corresponds with the
obvious external terminal groove.
The vestibule
supports the leaflets of the tricuspid valve and characteristically in
the right atrium it is surrounded by the pectinate muscles of the
appendage. At the first sight, the septal surface is extensive
but this is deceptive.
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The true site
of the septum is the location of the oval fossa. This structure marks
the position during fetal life where the richly oxygenated blood
coming to the baby from the placenta crossed over into the systemic
circuit so as to be distributed to the brain.
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During
intrauterine life, the blood for the growing fetus is oxygenated in
the maternal placenta rather than in the fetal lungs.
An essential
part of the atrial anatomy is a communication between the right and
left sides. This is found directly adjacent to the mouth of the
inferior caval vein, which is the portal of venous entry from the
placenta.
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The
communication in the fetus is called the oval foramen (foramen ovale).
It is designed so that it can close at birth when the lungs take over
the role of oxygenation. It has an obvious rim together with a floor,
the flap valve. The valve is hinged from the rim at its bottom edge,
where the rim separates the foramen from the coronary sinus and the
vestibule. This part of the atrial wall is called the sinus septum.
In fetal
life, a prominent fibromuscular fold directs the blood from the
inferior caval vein into the oval foramen.
This often
persists in postnatal life and is the Eustachian valve.
A similar
valve during fetal life, marking the junction between venous component
and appendage guards the opening of the coronary sinus and if this
persists after birth it is called the Thebesian valve.
In normal
hearts, the flap valve in the floor of the oval fossa is of sufficient
size to overlap the edges of the foramen. During fetal life its
upper margin is free, permitting the oxygenated blood to cross the
septum into the left atrium.
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At birth, with
onset of respiration, the pressure rises in the left atrium and the
flap valve is pressed against the rim of the oval fossa, thus closing
the communication. It is often stated that the rims of the fossa
itself are also extensive septal structures - the so-called "septum
secundum". (The flap valve itself is derived from the
primary atrial septum - the so-called septum primum ).
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Sectioning the
heart shows that the rim of the fossa for most of its extent is simply
formed from infoldings of the walls of the atriums. In normal
development, the upper margins of the flap valve attach by two horns
of the left atrial surface of the septum. With growth, the valve and
rim fuse together between these points.
In up to
one-third of the population, however, the edges appose rather than
fuse, giving a probe patent oval foramen. This is of no consequence as
long as left atrial pressure remains higher than right , but the
foramen can permit shunting should right atrial pressure exceed left,
as it can do in association with some congenital or acquired diseases
producing pulmonary hypertension. Visit:
Congenital Heart Disease
THE LEFT ATRIUM:
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The structure
of the left atrium though simpler than that of the right, still has
three components. The venous component is the dominant part
receiving one of the four pulmonary veins at each of its corners.
The appendage
is small with a characteristic tubular appearance and a narrow
junction with the venous component.
The pectinate
muscles are confined within the appendage and there is no terminal
crest marking the junction with the venous component.
The vestibule
of the mitral valve, therefore, is largely continuous with the
smooth- walled venous component.
The septal
surface of the left atrium is marked by the location of the flap
valve, this being most obviously seen where its two horns mark the
usual site of fusion with the rim. The anterior wall of the atrial
roof is often additionally corrugated at this site.
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