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THE
RIGHT VENTRICLE :
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The ventricles
have three components - the inlet, the apical trabecular component,
and the outlet.
The inlet of
the right ventricle extends from the atrio-ventricular junction ,
being contained and limited by the tension apparatus of the tricuspid.
The tension
apparatus stops the atrio-ventricular valves from turning inside-out
during ventricular contraction.
The three
leaflets of the tricuspid valves - Septal ; Antero-superior ; and
Inferior or mural location. The medial
papillary muscle supports the zone of apposition between septal and
antero-superior leaflets.
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It is these
zones of apposition between adjacent leaflets which we define as the
commissures of the valve.
A much larger
papillary muscle, the anterior papillary muscle, supports the
commissure between antero-superior and inferior leaflets, although
this muscle often inserts along the edge of the extensive antero-superior
leaflet.
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The third
commissure, between inferior and septal leaflets, is supported by the
smaller inferior papillary muscle.
The most
characterisitic feature of the tricuspid valve is the multiple
attachment of cords from the septal leaflet to the ventricular septum.
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The
characterisitic feature of the apical component of the ventricle is
its very coarse trabeculations.
The very
prominent septomarginal trabeculation reinforces the surface of the
septum and divides in the roof of the ventricle into two limbs which
clasp the infolding of the heart wall forming the ventricular roof.
This infoldings is the supra-ventricular crest. It is the part
of the ventricle which wraps itself around the origin of the aorta
from the left ventricle. The posterior
limb of the septomarginal trabeculation gives rise to the medial
papillary muscle, while the anterior limb runs into the outlet
component to support the leaflets of the pulmonary valves.
The body of
the trabeculation then runs down towards the apex of the right
ventricle, where it gives rise to the anterior papillary muscle,
before breaking up into the general apical trabeculations .
The anterior
papillary muscle itself continues as an important muscular strand, the
moderator band, to the parietal wall of the right ventricle.
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A further
series of trabeculations run from the anterior surface of the
septomarginal trabeculation onto the parietal wall of the right
ventricle. These are septoparietal trabeculations. The major
feature of the outlet of the right ventricle is that it is a complete
muscular structure. Known as the
infundibulum, it can be lifted clear of the heart without entering the
cavity of the left ventricle.
It supports
the three leaflets of the pulmonary valve in semilunar fashion.
Each of the
leaflets is attached such that the base is within the ventricle while
the tip of the zone of apposition takes origin from the prominent
junction between the sinuses of the pulmonary trunk and the tubular
arterial trunk.
Each semilunar
attachment, therefore, marking the haemodynamic ventriculo-arterial
junction, crosses the circular line of attachment of the fibroelastic
pulmonary trunk to the muscular right ventricular infundibulum. This
arrangement permits the valvar leaflets to open and close in competent
fashion. The zones of apposition of the leaflets, or commissures,
extend from the peripheral attachment at the sinutubular junction to
the centre of the valve.
This mechanism
is also seen within the aortic valve. The understanding of the
arrangement is important because of the frequent use of the term
"annulus" when describing the arterial valvar complex. None
corresponds to the semilunar attachment of the leaflets. This is
unlike the arrangement in the atrio-ventricular valves where the
leaflets are attached at the atrio-ventricular junctions in true
annular fashion.
THE LEFT VENTRICLE
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The left
ventricle has the mitral valve within its inlet component, has fine
apical trabeculations with a smooth septal surface, and has the aortic
valve in its outlet component.
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The most
characterisitic feature of the normal ventricle is the overlapping of
the inlet and outlet components, with fibrous continuity between the
leaflets of the mitral and aortic valves in the ventricular roof.
The mitral
valve's old name was the bicuspid valve, thus distinguishing it from
the atrio-ventricular valve of the right heart with its three
leaflets.
The mitral valve
has two leaflets and these are markedly dissimilar in their structure.
One leaflet
guards one-third of the valvar circumference and is deep. The other is
much shallower and guards along its length known as "scallops". The
leaflets are often said to be "anterior" and "posterior". When viewed
with the heart in position within the body, the valvar orifice is
markedly oblique and these terms are not strictly correct. Some
prefer to describe them as the mural and aortic leaflets.
A valve with two
leaflets can have only one zone of apposition between them. The mitral
valve, has only one commissure. This extends from antero-laterally
to postero-medially within the atrio-ventricular junction.
Since
pathologists tend to view the valve when opened, they more usually
describe the two ends of the zone of apposition as paired commissures
within the valve.
The tension
apparatus from both sides of the ends of adjacent leaflets are
attached to the paired papillary muscles, also located antero-laterally
and postero-medially within the ventricular mass. The two muscles are
positioned directly beneath the ends of the commissure so as to act at
the maximal mechanical efficiency.
In the normal
valve, all parts of the leaflets usually have cords connecting
the free edge to the papillary muscle. Two cords, on the ventricular
aspect of the aortic leaflet, are particularly prominent and are
called the strut cords. Fan-shaped cords are found at the ends of the
zone of apposition between the leaflets and also between the scallops
of the mural leaflet. Other shorter cords, basal cords, run directly
from the undersurface of the mural leaflet to the parietal ventricular
wall. Similar variation in cordal morphology is also seen in the
tricuspid valve.
The
interrelationship of aortic and mitral valves in the roof of the
ventricle have an important consequence on septal anatomy.
The larger
part of the interventricular septum is a muscular structure.
Because of the
overlapping of the inlet by the outlet of the left ventricle, however,
the outlet of the left ventricle is adjacent to the inlet of the
right. This can be readily demonstrated by removing one of the
sinuses of the aortic valve. It can then be seen that the
rightward margin of the area of continuity between the two sides of
the heart between the leaflets of the tricuspid valve.
The
intervening area, composed of fibrous tissue is incorporated as part
of the septum between the two sides of the heart. This fibrous
component is called the membranous septum.
It is itself
divided by the attachment of the tricuspid valve on its right side
into one part between the ventricles (interventricular membranous
septum) and a second part interposed between right atrium and
subaortic outflow tract (the atrioventricular membranous septum). This
part of the septum achieves added importance since it is pierced by
the axis of atrioventricular conduction tissue (the penetrating
atrioventricular bundle or bundle of His).
The aortic valve
itself is arranged in a fashion similar to that described for
pulmonary valve. Its three leaflets have semilunar attachments which
cross the anatomic ventriculo-arterial junction.
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Unlike the
pulmonary valve, these leaflets are not exclusively supported by
ventricular musculature. Two of the leaflets do have such muscular
support ,these being the ones adjacent to, or facing, the leaflets of
the pulmonary valve. These facing leaflets guard the aortic sinuses
which give rise to the coronary arteries and hence are called the
right coronary and left coronary leaflets. The third leaflet, guarding
the sinus which does not give rise to a coronary artery, is the
non-coronary leaflet.
It is parts of
the non-coronary and left coronary leaflets which are in fibrous
continuity with the aortic leaflet orf the mitral valve, this
important area is also known as the aorto-mitral curtain.
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