pediagenosis
November 07, 2024
AnatomyPhysiology
,
Cardiovascular
,
Organ
,
science
,
Thorax
Comment
ATRIA AND
VENTRICLES
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| RIGHT ATRIUM AND RIGHT VENTRICLE |
The right atrium consists of two parts: (1) a posterior smooth-walled part derived from the embryonic sinus venosus, into which enter the superior and inferior venae cavae, and (2) a thin walled trabeculated part that constitutes the original embryonic right atrium. The two parts of the atrium are separated by a ridge of muscle. This ridge, the crista terminalis (see Plate 1-7), is most prominent superiorly, next to the SVC orifice, then fades out to the right of the IVC ostium. Its position corresponds to that of the sulcus terminalis externally (see Plate 1-6). Often described as a remnant of the embryonic right venous valve. the crista terminalis actually lies just to the right of the valve.
From the lateral aspect
of the crista terminalis, a large number of pectinate muscles run
laterally and generally parallel to each other along the free wall of the
atrium. The atrial wall is paper-thin and translucent between the pectinate
muscles. The triangular-shaped superior portion of the right atrium the right auricle is also filled with pectinate
muscles. One pectinate muscle originating from the crista terminalis is usually
larger than the others and is called the taenia sagittalis.
The right auricle usually
is not well demarcated externally
from the rest of the atrium. The right auricle is a convenient, ready-made
point of entry for the cardiac surgeon and is used extensively.
The
anterior border of the IVC ostium is guarded by a fold of tissue, the inferior
vena cava (eustachian) valve, which varies greatly in size and may
even be absent. When large, the IVC valve is usually perforated by numerous
openings, forming a delicate lacelike structure known as the network of Chiari.
The coronary sinus enters the right atrium just anterior to the medial
extremity of the IVC valve. The eustachian valve’s orifice may also be
guarded by a valvelike fold, the coronary sinus (thebesian) valve.
Both IVC valves and coronary sinus valves are derived from the large, embryonic
right venous valve.
The posteromedial wall of
the right atrium is formed by the interatrial septum, which has a thin,
fibrous, central ovoid portion. The interatrial septum forms a shallow
depression in the septum called the fossa ovalis. The remainder of the
septum is muscular and usually forms a ridge around the fossa ovalis, the limbus
fossae ovalis. A probe can be passed under the anterosuperior part of the
limbus into the left atrium in some cases, and the foramen (fossa) ovalis is
then “probe patent.” Anteromedially, the tricuspid valve gives access to the
right ventricle.
The right ventricular
cavity (see Plate 1-7) can be divided arbitrarily into a
posteroinferior inflow portion, containing the tricuspid valve, and an
anterosuperior outflow portion, from which the pulmonary trunk originates.
These two parts are separated by prominent muscular bands, including the parietal
band, the supraventricular crest (crista supraventricularis), the septal
band, and the moderator band. These bands form a wide, almost
circular orific with no impediment to flow in the normal heart.
The wall of the inflow
portion is heavily trabeculated, particularly in its most apical portion. These
trabeculae carneae enclose a more or less elongated, ovoid opening. The
outflow portion of the right ventricle, often called the infundibulum, contains
only a few trabeculae. The subpulmonic area is smooth walled.
A number of papillary
muscles anchor the tricuspid valve cusps to the right ventricular
wall through many slender, fibrous strands called the chordae tendineae. Two
papillary muscles, the medial and anterior, are reasonably constant in
position but vary in size and shape. The other papillary muscles are extremely
variable in all respects. Approximately where the crista supraventricularis
joins the septal band, the small medial papillary muscle receives
chordae tendineae from the anterior and septal cusps of the tricuspid valve.
Often well developed in infants, the medial papillary muscle is almost absent
in adults or is reduced to a tendinous patch. An important surgical landmark,
the medial papillary muscle is also of diagnostic value to the cardiac
pathologist with its interesting embryonic origin. The anterior papillary
muscle originates from the moderator band and receives chordae from the
anterior and posterior cusps of the tricuspid valve. In variable numbers, the
usually small posterior papillary muscle and septal papillary muscle
receive chordae from the posterior and medial (septal) cusps. The muscles
originating from the posteroinferior border of the septal band are important in
the analysis of some congenital cardiac anomalies.
The pulmonary trunk arises
superiorly from the right ventricle and passes backward and slightly upward. It
bifurcates into right and left pulmonary arteries (see Plate 1-7) just
after leaving the pericardial cavity. A short ligament the ligamentum
arteriosum (see Plate 1-8) connects the upper aspect of the bifurcation to the inferior
surface of the aortic arch (arch of aorta; see Plate 1-6). It is a remnant of the fetal ductus arteriosus (duct of Botallo).
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| LEFT ATRIUM AND LEFT VENTRICLE |
Left Atrium
The left atrium consists
mainly of a smooth-walled sac with the transverse axis larger than the vertical
and sagittal axes. On the right, two or occasionally three pulmonary veins enter
the left atrium; on the left there are also two (sometimes one) pulmonary
veins. The wall of the left atrium is distinctly thicker than that of the right
atrium. The septal surface is usually fairly smooth, with only an irregular
area indicating the position
of the fetal valve of the foramen ovale. A narrow slit may allow a probe
to be passed from the right atrium to the left atrium.
The left auricle is
a continuation of the left upper anterior part of the left atrium. The
auricle’s variable shape may be long and kinked in one or more places. Its
lumen contains small pectinate muscles, and there usually is a distinct
waistlike narrowing proximally.
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| ATRIA, VENTRICLES, AND INTERVENTRICULAR SEPTUM |
The left ventricle (see
Plate 1-8) is egg shaped with the blunt end cut off, where the mitral valve and
aortic valve are located adjacent to each other. The valves are
separated only by a fibrous band giving off most of the anterior (aortic)
cusp of the mitral valve and the adjacent portions of the left and
posterior aortic valve cusps. The average thickness of the left
ventricular (LV) wall is about three times that of the right ventricular (RV) wall.
The LV trabeculae carneae are somewhat less coarse, with some just tendinous
cords. As in the right ventricle, the trabeculae are much more numerous and dense in the apex of the left ventricle. The
basilar third of the septum is smooth.
Usually there are two
stout papillary muscles. The dual embryonic origin of each is often
revealed by their bifid apices; each receives chordae tendineae from both
major mitral valve cusps. Occasionally a third, small papillary
muscle is present laterally.
Most of the ventricular
septum is muscular. Normally it bulges into the right ventricle, showing
that a transverse section of the left ventricle is almost circular. The muscular
portion has approximately the same thickness as the parietal LV wall. The
ventricular septum consists of two layers, a thin layer on the RV side and a
thicker layer on the LV side. The major septal arteries tend to run between
these two layers. In the human heart a variable but generally small area of the
septum immediately below the right and posterior aortic valve cusps is
thin and membranous.
The demarcation between
the muscular and the membranous part of the ventricular septum is
distinct and is called the limbus marginalis. As seen from the opened right ventricle (see Plate
1-7, bottom), the membranous septum lies deep to the supraventricular
crest and is divided into two parts by the origin of the medial (septal)
cusp of the tricuspid valve. As a result, one portion of the
membranous septum lies between the left ventricle and the right ventricle the interventricular part and the other between the left
ventricle and the right atrium the
atrioventricular part.
On sectioning of the
septum in an approximately transverse plane, the basilar portion of the
ventricular septum, including the membranous septum, is seen to deviate to the
right, so that a plane through the major portion of the septum bisects the aortic
valve. It must be emphasized that the total cardiac septum shows a complex,
longitudinal twist and does not lie in any single plane.
