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Cardiovascular System
Development of the Vascular System
Development of the vascular system begins in the wall of the yolk sac during the third week
of gestation (18 days) with the formation of blood islands. At this time the embryo is too
large for distribution of oxygen to all tissues by diffusion alone.
The precursor cells of blood islands are hemangioblasts and they have the capacity to
give rise to either endothelial cells or hematopoietic cells. Endothelial cells arise from
angioblasts and hematopoietic cells arise from hemocytoblasts. Once committed to one of
the two lineages, daughter cells of hemangioblasts lose the capacity to form other cells types.
Very early in development, the line of active blood-forming cells (hemocytoblasts, otherwise
known as pluripotent cells) subdivides into two separate lineages. [1] Lymphoid stem cells
ultimately form two lines of lymphocytes; B lymphocytes and T lymphocytes and [2]
Myeloid stem cells, which are precursors to other lines of blood cells; erythrocytes,
granulocytes (neutrophils, eosinophils, and basophils), monocytes, and platelets.
The yolk sac blood islands are a temporary provider of hematopoiesis. Soon, other
intraembryonic sites will form blood islands. At 28 days, definitive intraembryonic
hematopoiesis begins in small clusters of cells in the splanchnopleuric mesoderm associated
with the ventral wall of the dorsal aorta called paraaortic clusters. By six to eight weeks
gestation the liver replaces the yolk sac as the main source of blood cells. Red blood cells
produced in the liver are larger than adult RBC's, are nucleated, and have different types of
hemoglobin. The fetus has hemoglobin (Hb) of the α2γ2 form and adult Hb is predominately
of the α2β2 form. Fetal Hb gamma (γ2) chains do not bind to 2,3 diphosphoglycerate, thus
fetal Hb has a higher affinity for oxygen. This aids in the transfer of oxygen from mother to
fetus in the placenta. At six months of pregnancy, liver production of RBC's declines and
their production shifts to bone marrow derived formation.
Blood Vessel Formation
The early embryo is devoid of blood vessels. The first patterns of a population of vascular
precursors are the hemangioblasts. These cells, in part, give rise to angioblasts and organize
into a primary capillary plexus through a process known as vasculogenesis. Subsequent
resorption of existing vessels and sprouting of new ones is called angiogenesis.
Angiogenesis continues throughout adult life. Comprehensive studies show that angioblasts
arise from most mesodermal tissue of the body except notochord and prechordal
mesoderm.
Formation of the Heart Tube
Cardiac progenitor cells are derived from intraembryonic mesoderm emerging from the
cranial third of the primitive streak during gastrulation. The cardiac progenitor cells
eventually become localized within the cranial lateral plate mesoderm on both sides of the
embryo, forming the cardiac crescent or cardiogenic plate. This plate is rostral to the
buccopharyngeal membrane.
Lateral plate mesoderm is subdivided into somatic and splanchnic mesoderm. The
splanchnic portion has the capacity to develop into cardiac cells. As the two limbs of the
cardiac crescent fuse, a pair of vascular elements, endocardial tubes, develops within each
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limb of the cardiac crescent. It is thought that cranial endoderm has vascular endothelial
growth factor (Vegf) that directs certain mesodermal cells in the cardiac crescent to develop
into endothelial / endocardial cells. The endocardial tubes coalesce into a single tube to
make a primitive heart tube.
The primitive heart tube has progenitors for the atria and ventricles, as well as
endocardium. A thick layer of extracellular matrix, the cardiac jelly, is deposited mainly by
the developing myocardium, separating it from the fused endocardial cells. From inside to
outside, the primary heart tube is composed an endocardial tube (endocardium), cardiac jelly,
and a myocardial tube( cardiac muscle). The epicardium is derived from mesodermal cells
that independently migrated from the splanchnic mesoderm.
The inflow end (sinus venosus) begins as the confluence of the left and right sinus horns.
The sinus horns receive the common cardinal veins. Cranial to the sinus venosus are the
primitive atrium and then the primitive ventricle. These are separated from each other by
the atrioventricular sulcus. The primitive atrium will form both the left and right atria and
the primitive ventricle will form the definitive left ventricle. The primitive ventricle is
separated by the next part of the heart tube, the bulbus cordis, by the bulboventricular
sulcus. The bulbus cordis will form much of the right ventricle. Lastly, the most cranial
segment, the outflow tract (or conotruncal segment) forms the distal outflow region for
both the left and right ventricles. This conotruncus can be subdivided into two parts.
(1) The proximal part has a conus cordis, which is the outflow tract for both ventricles.
When the heart is partitioned into four chambers, the conus cordis ceases to exist. However,
that part of it that remains in the right ventricle is called the conus arteriosus (leads to
pulmonary trunk). (2) The other part is the truncus arteriosus, which eventually splits into
the ascending aorta and pulmonary artery. The truncus arteriosus ends with an aortic sac.
The aortic sac gives off the pairs of 5 aortic arches.
The primitive atrium gives rise to the pectinate muscles of the atrial auricles. The bulbus
cordis and primitive ventricle gives rise to the trabeculae carnae in the ventricles.
By 22 days after fertilization, cardiac muscle cells have sufficiently advanced to
allow the heart to begin beating.
Cardiac Looping
On day 23, the primitive heart tube begins to elongate and simultaneously bend into an
S-shaped structure. The primitive heart is initially suspended by a dorsal mesocardium.
The dorsal mesocardium quickly ruptures leaving the heart suspended in the pericardial
cavity by its attached vasculature. The region of the ruptured dorsal mesocardium becomes
the transverse pericardial sinus. As the heart elongates, the bulbus cordis is displaced to
the right (caudally and ventrally); the primitive ventricle is to the left of the bulbus cordis
(right ventricle); and the primitive atrium is positioned dorsally and cranially.
Development of the Venous Sinuses
In the fourth week the sinus venosus receives blood from the right and left sinus horns.
Each horn receives blood from the (1) vitelline or omphalomesenteric vein, (2) umbilical
vein, and (3) common cardinal vein. In the fifth week, the left sinus horn diminishes. At
the heart, the right and left umbilical and left vitelline veins disappear. The right vitelline
vein will help form the inferior vena cava. The anterior and posterior cardinal veins are
tributaries of the common cardinal vein. At ten weeks the left common cardinal vein
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disappears, hence the left sinus horn shrinks down into what remains as the oblique vein of
the left atrium and the coronary sinus. As a result, the increased flow causes the right
sinus horn and veins to greatly enlarge. Also, there is a right to-left shunt being formed
within the heart which further increases the vessels in the right side of the developing heart
The right horn is integrated into the right atrium forming the smooth wall of the right
atrium. At the entrance of the sinus into the right atrium, there is an opening called the
sinuatrial orifice that is flanked on both sides by the right and left venous valves. The
upper parts of the valves fuse inside the atrium, forming a ridge, the septum spurium.
While the superior portion of the right venous valve disappears, its inferior portion forms the
valve of the inferior vena cava and valve of the coronary sinus. The crista terminalis
forms the dividing line between the original trabeculated right atrium and the smooth-walled
part that was contributed by the sinus horn, also called the sinus venarum. The sulcus
terminalis is external to the crista terminalis.
Fetal Blood Flow to the Heart and the Right–To-Left Shunt: Vitelline and umbilical
veins begin as pairs of symmetrical vessels that drain separately into the venous sinuses of
the heart. Overtime, these vessels are also associated with the liver. The vitelline veins,
which drain the yolk sac, develop sets of anastomosing channels both within and outside the
liver. Outside the liver the two vitelline veins and their side-to-side anastomotic channels
become closely associated with the duodenum. Through the persistence of some vitelline
channels and the disappearance of others, the hepatic portal vein, which drains the
intestines, takes shape. The superior mesenteric vein derives from the right vitelline vein.
At the liver, the left vitelline vein regresses. The right vitelline vein takes on importance as
the hepatocardiac portion of the inferior vena cava. Oxygenated blood from the placenta
flows through the left umbilical vein. From the left umbilical vein, blood flows into the right
hepatocardiac channel (ductus venosus) and the then inferior vena cava, both derived from
the right vitelline vein. From the hepatic capillary bed, the blood that arrives from the
hepatic portal vein passes into a set of hepatic veins, which empty into this part of the
inferior vena cava blood, then into the sinus venosus.
The original the symmetrical umbilical veins soon lose their own hepatic segments and
drain directly into the liver by combining with the intrahepatic vascular plexus of vitelline
veins. Soon, a major channel, the ductus venosus, forms and shunts much of the blood
entering from the left umbilical vein directly through the liver and into the inferior vena cava.
Before long, the right umbilical vein degenerates, leaving the left umbilical vein the sole
channel for bring blood that has been reoxygenated and purified in the placenta back to the
embryonic body. The ductus venosus permits the incoming oxygenated placental blood to
bypass the capillary networks of the liver so the refreshed blood can enter the right side of
the heart. From the right side of the heart, the oxygenated blood passes from the right atrium
to the left atrium via the foramen ovale. Any blood that enters the right ventricle will also
bypass the lungs via the ductus arteriosus, which empties the descending aorta. Therefore,
this right-to-left detour shunts blood away from the lungs during fetal development.
Early Partitioning of the Heart
Early in heart development the atrium becomes partially separated from the ventricle by
formation of thickened atrioventricular cushions. Endocardial cells in the areas where
endocardial cushions form, are stimulated to proliferate and cross the cardiac jelly. Only in
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the areas of the endocardial cushions are the endocardial cells stimulated by the overlying
myocardium. They accumulate against the inside of the myocardium, thus producing
partitions in the heart. The endocardial cushions function as primitive valves that assist in
forward propulsion of blood. Mature heart valves develop at these cushions. Disturbances in
molecular induction to produce these processes can account for many malformations of the
heart.
Later Partitioning of the Heart
Separation of the Atria from the Ventricles
On the dorsal and ventral walls of the atrioventricular canal, the endocardial cushions
transform into dense connective tissue. As they grow into the canal, the two cushions meet
and separate the atrioventricular canal into right and left channels. Later development results
in the formation of thin leaflets, which are to become the anatomical valves. The definitive
atrioventricular valve leaflets are also derived from invaginated epicardial cells from the
atrioventricular groove. The right atrioventricular valve is the tricuspid valve and the left
one is the bicuspid or mitral valve.
In the left and right ventricles, the valves are covered by endocardium. They are
connected to thick trabeculae called papillary muscles by means of chordae tendineae.
Partitioning of the Atria
While the atrioventricular canals are taking shape, the common atrium is being separated into
left and right chambers. Partitioning begins in the fifth week with the downgrowth of a
crescentic interatrial septum primum. The apices of the crescent of the septum primum
extend toward the atrioventricular canal and merge with the endocardial cushions. The space
between the leading edge of the septum primum and the endocardial cushions is called the
interatrial foramen (ostium) primum. Maintaining a balanced circulatory load on all
chambers is met by the existence of the two shunts that bypass the lungs, the foramen ovale
and ductus arteriosus.
When the interatrial septum primum is almost ready to fuse with the endocardial cushions,
an area of genetically programmed death causes the appearance of multiple perforations near
its cephalic end. As the leading edge of the septum primum fuses with the endocardial
cushions, hence obliterating the ostium primum, the perforations coalesce and give rise to the
interatrial foramen (ostium) secundum. The ostium secundum preserves the connection
between the right and left atria.
Shortly after the appearance of the ostium secundum, a crescentic septum secundum
begins to form just to the right of the septum primum. This structure, which grows out from
the ventral part of the atrium, forms the foramen ovale. The position of the foramen ovale
allows most of the blood entering the right atrium from the inferior vena cava to pass directly
through it and foramen secundum into the left atrium. The valve of the foramen ovale is the
flap of tissue that covers its left side produced by the septum primum. This arrangement
allows blood to flow from the right to left atrium, but not the reverse.
Repositioning of the Sinus Venosus and the Venous Inflow into the Atria
As the heart undergoes looping and the interatrial septa form, the entrance of the sinus
venosus shifts completely to the right atrium. In tandem, the right horn of the sinus venosus
becomes increasingly incorporated into the wall of the right atrium and the left horn is
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reduced to the coronary sinus. At the entrances of the superior and inferior vena cavae,
valvelike flaps appear: the venous valves or valvulae venosae.
Regarding the left atrium, the pulmonary vein develops an outgrowth of the posterior left
atrial wall. This vein gains connection with veins of the developing lung buds. During
further development the pulmonary vein forms the smooth wall of the adult left atrium.
Ultimately, four veins enter the left atrium. The embryonic atrium forms the trabeculated
appendage of both atria. On the right, the trabeculated part has the pectinate muscles.
Partitioning of the Ventricles
When the interatrial septa are first forming, a muscular interventricular septum begins to
grow from the apex of the common ventricle toward the atrioventricular endocardial
cushions. The early division of the common ventricle is reflected by the presence of a
groove on the outer surface of the heart. Although the interventricular foramen is initially
present, it is ultimately obliterated. this is accomplished by (1) further growth of the
muscular interventricular septum, (2) a contribution by conotruncal ridge tissue that divides
the outflow of the heart, and (3) a membranous component derived from endocardial cushion
connective tissue.
Partitioning of the Outflow Tract of the Heart
In the very early tubular heart the outflow tract is a single tube, the bulbus cordis. When the
interventricular septum begins to form, the bulbus elongates and is divided into a proximal
conus arteriosus and a distal truncus arteriosus. Initially, the outflow tract (truncus
arteriosus) is a single channel, but then it begins to be partitioned into separate aortic and
pulmonary channels as a result of the appearance of two spiral conotruncal ridges. These
ridges bulge into the lumen and finally meet, thus forming the two channels. The aortic sac,
which is located distal to the conotruncal region, does not contain ridges. Partitioning begins
near the ventral aortic root between the fourth and sixth arches and extends toward the
ventricles, spiraling as it goes. This accounts for the partial spiraling of the aorta and the
pulmonary artery in the adult heart.
At the base of the conus, where endocardial cushion tissue is formed in the same manner
as in the atrioventricular canal, two new sets of semilunar valves form. These valves have
three leaflets each, preventing blood from washing back into the ventricles. Cranial neural
crest cells and cardiac mesoderm contribute to the formation of semilunar valves. Just past
the aortic side of the aortic semilunar valve, the two coronary arteries join the aorta to
supply the heart muscle. The coronary arteries arise from splanchnic mesoderm that also
forms the epicardium. The coronary then vessels migrate and connect with the aorta
Development of Arteries
Aortic Arches
When pharyngeal arches form during the fourth and fifth weeks, each arch receives its own
cranial nerve and artery or aortic arch. In human embryos, all aortic arches are never
present at the same time. With the conotruncal tract now separated into aortic and pulmonary
arteries, the aorta empties into the aortic sac. The aortic arches branch off from the aortic
sac. There are five arches numbered I, II, III, IV, and VI: arch V does not develop well. The
aortic sac forms right and left horns that give rise to the brachiocephalic and proximal
aortic arch.
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By day 27 the first aortic arch has mostly disappeared, but persists as the maxillary
artery.
Likewise, the second aortic arch vanishes and its remains are the hyoid and stapedial
arteries.
The third aortic arch forms the common carotid artery and the first part of the internal
carotid artery. The remainder of the internal carotid artery is formed by the cranial portion
of the dorsal aorta. The external carotid artery is a sprout of the third aortic arch.
The fourth aortic arch continues on both sides. However, on the left side it forms part of
the aortic arch between the left common carotid and the left subclavian arteries. On the right
side, it forms most of the proximal segment of the right subclavian artery. The distal of the
right subclavian artery has contributions from the right dorsal aorta and 7th intersegmental
artery.
The fifth aortic arch never forms completely and regresses.
The sixth aortic arch or pulmonary arch, gives off a branch to the developing lung bud.
On the right side, the proximal part becomes the proximal segment of the right pulmonary
artery while its distal part disappears. On the left side, it is the proximal part of the left
pulmonary artery, but the distal part persists during intrauterine life as the ductus arteriosus.
As a result of the caudal shift of the heart and the disappearance of various portions of the
aortic arches, the course of the recurrent laryngeal nerves (vagus) becomes different on
both sides. Initially, these nerves supply the sixth pharyngeal arches. As the heart descends
caudally, they are anterior to the 6th aortic arches, then hook under and posterior to the
arches, ascending to the larynx. On the right side, when the 5th and 6th aortic arches
disappear, the recurrent laryngeal nerve is hooked under the right subclavian artery (4th arch).
On the left side, the recurrent laryngeal nerve does not move up since the distal part of the
sixth aortic arch persists as the ductus arteriosus and the nerve is hooked beneath it. After
birth the ductus arteriosus is transformed into the ligamentum arteriosum.
Vitelline and Umbilical Arteries
The paired vitelline arteries supply the yolk sac. Gradually, they fuse to form arteries in the
dorsal mesentery of the gut. These are the celiac, superior mesenteric, and inferior
mesenteric arteries. These vessels supply the foregut, midgut, and hindgut, respectively.
The umbilical arteries are branches of the common iliac arteries: paired intersegmental
branches off the dorsal aorta that supply the leg buds. They course to the placenta to
reoxygenate the blood. After birth their proximal portions persist as the internal iliac and
superior vesicle arteries. Their distal parts obliterate to form the medial umbilical
ligaments.
Arteries of the Head
Ventrally, the aortic arch system (1st to 3rd arches) gives rise to arteries supplying the face
(external carotid artery) and the frontal part of the base of the brain (internal carotid
arteries).
The vertebral arteries (from intersegmental arteries) grow toward the brain. Soon they
merge at the midline to form the basilar artery. As the basilar artery approaches the
diencephalon it gives off sets of arteries of which the posterior communicating arteries is
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one and those connect to the internal carotid arteries. Anteriorly, branches off the internal
carotid artery fuse in the midline and a vascular ring is formed: the circle of Willis. The ring
surrounds the diencephalon, optic chiasm, and pituitary stalk.
Development of Veins
Common Cardinal Veins
The cardinal veins form the basis for the intraembryonic venous system. The earliest of
cardinal veins consists of paired anterior and posterior cardinal veins, which drain blood
from the head and body into short common cardinal veins. The common cardinal veins
empty their blood into the sinus venosus of the primitive heart.
In the cranial region the symmetrical anterior cardinal veins are transformed into the
internal jugular veins. As the heart rotates to the right, the base of the left internal jugular
vein is attenuated. At the same time a new anastomotic channel (future left brachiocephalic
vein) connects the left internal jugular vein with the right one. Through this anastomosis,
blood from the left side of the head is drained into the original right anterior cardinal vein
(future superior vena cava), which drains into the right atrium. In the meantime, while the
left common cardinal vein is regressing, its proximal portion becomes the coronary sinus.
In the trunk a pair of subcardinal veins arises is association with the developing
mesonephros (urogenital precursors). Anastomosis between the left and right subcardinal
veins forms the left renal vein. The left subcardinal vein disappears except for its distal
portion, which becomes the left gonadal vein. The right subcardinal vein forms the right
renal vein and is also makes the renal portion of the inferior vena cava draining the left and
right kidneys (renal section). Soon, a pair of supracardinal veins appears in the body wall,
dorsal to the subcardinal veins. The supracardinal veins drain the body wall (intercostal
spaces and lumbar area). Eventually, parts of the supracardinal veins in the abdomen will
regress, but in the thorax they will persist with this general description. The left
supracardinal vein will form the hemiazygos vein (drains caudal four or five intercostal
veins and crosses about T9 vertebral level to the azygos vein) and the accessory hemiazygos
vein (drains the 4th through 7th intercostal space and crosses about T8 to azygos vein). The
right supracardinal vein will drain the 4th through 11th intercostal spaces as the azygos vein.
Its caudal portion persists between the renal section of the inferior vena cava (IVC) and the
sacrocardinal veins.
The sacrocardinal veins drain the lower extremities. The sacrocardinal veins drain into
the supracardinal veins. In the abdomen, the left supracardinal vein vanishes, but by this time
an anastomotic channel has formed, linking the right and left sacrocardinal veins. This
anastomosis between the sacrocardinal veins forms the left common iliac vein. The right
sacrocardinal vein becomes the sacrocardinal segment of the inferior vena cava and the right
common iliac vein.
In short, the subcardinal, sacrocardinal, and supracardinal veins drain, respectively,
the kidneys, the lower extremities, and the body wall. In the thoracic region, the right
supracardinal vein forms the azygos vein and part of IVC The left supracardinal vein
forms the hemiazygos and accessory hemiazygos veins and main tract of the left
superior intercostal vein.
If you can see in the section of this chapter, "Fetal Blood Flow to the Heart and the Right–
To-Left Shunt," the proximal part of the inferior vena cava draining directly into the right
atrium is derived from the right vitelline vein.
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Malformations of the Heart
Clinically, heart malformations are typically associated with cyanosis in post natal life and
those that are acyanotic. Cyanosis is readily recognized by a purplish to bluish tinge of the
skin. It is associated with polycythemia (increased red blood cell count). Long-term
cyanosis is associated with prominent clubbing of the fingers and decreased growth.
Postnatally, cyanosis is associated with the presence of a right-to left shunt in which
venous blood mixes with systemic blood. Some heart defects are acyanotic for many years,
but then become cyanotic. These defects are initially characterized by a left-to-right shunt
where oxygenated systemic blood refluxes into the right atrium or ventricle. The net result is
an increased pumping load on the right ventricle, ultimately leading to right ventricular
hypertrophy. Over a long period the increased blood flow through the lungs provokes a
hypertensive reaction in the pulmonary vasculature, which effectively increases the pressure
in the right atrium and ventricle. When the blood pressure on the right side of the heart
exceeds that in the corresponding left chamber, the shunt reverses, and poorly oxygenated
blood passes to the systemic circulation, thus leading to cyanosis.
Chamber-To-Chamber Defects
Interatrial Septal Defects
The most common are caused by excessive resorption of tissue around the foramen
secundum or hypoplastic growth of the septum secundum. Less commonly, there is a lack of
union on the septum primum and endocardial cushions.
Uncomplicated atrial septal defects are usually compatible for many years of life. During
the symptom-free period, blood from the slightly higher pressure left atrium enters the right
atrium. Over many years, however, pulmonary hypertension can develop.
Interventricular Septal Defect
Defects in the interventricular septum are the most common congenital defect in infants, by a
vast majority of them close spontaneously by age 10. In the adult they are not as common as
atrial septal defects. Because the pressure in the left ventricle is higher than the right, it is
initially presented as a left-to-right shunt. However, after the increased blood flow causes the
right ventricle to hypertrophy, pulmonary hypertension ensues with a resultant reversal of
blood flow and associated cyanosis.
Malformations of the Outflow Tract
Persistent Truncus Arteriosus
Persistent truncus arteriosus is caused by the lack of partitioning of the outflow tract by the
truncoconal ridges. Because the ridges contribute to the membranous part of the
interventricular septum, this malformation is almost always associated with a ventricular
septal defect. A large arterial outflow vessel overrides the ventricular septum and receives
blood that exits each ventricle. As may be predicted, these infant individuals are highly
cyanotic with 65% dying in six months if there is no treatment.
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Transposition of the Great Vessels
On rare occasions the truncoconal ridges fail to spiral as they divide the outflow tract into
two channels. This results in two totally independent circulatory arcs. The right ventricle
empties into the aorta and the left ventricle empties into the pulmonary trunk. This lesion is
the most common cause of cyanosis in newborns. It is compatible with life only if an atrial
and ventricular septal defect and an associated patent ductus arteriosus accompany it.
Questions
1.
The mesodermal precursors that develop into vascular endothelial cells and blood cells
are which of the following?
a. hemangioblasts
b. hemocytoblasts
c. angioblasts
d. myeloid stem cells
2.
In the embryo, the first place that makes blood islands is the ___________.
a. bone marrow
b. liver
c. spleen
d. yolk sac
3.
For heart creation, which of the following correctly describes the cardiogenic plate?
a. It is U-shaped and caudal to the buccopharyngeal (oropharyngeal) membrane.
b. It is U-shaped and rostral to the buccopharyngeal (oropharyngeal) membrane.
c. It is U-shaped and caudal to the cloacal membrane.
d. It is U-shaped and rostral to the cloacal membrane.
4.
The myocardium is derived from ___________ mesoderm.
a. somatic
b. paraxial
c. splanchnic
d. intermediate
5.
Which of the following develops within the cardiac crescent, fuses, and then forms the
primitive heart tube?
a. sinus horns
b. cardiac jelly
c. endocardial tubes
d. truncus arteriosus
6.
Which of the following is in the correct order, from input to outflow?
a. truncus arteriosus > bulbus cordis > primitive ventricle > primitive atrium
b. bulbus cordis > primitive ventricle > primitive atrium > truncus arteriosus
c. primitive ventricle > primitive atrium > truncus arteriosus > bulbus cordis
d. primitive atrium > primitive ventricle > bulbus cordis > truncus arteriosus
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7.
The conus arteriosus will become the outflow for the mature _________.
a. right atrium
b. right ventricle
c. left atrium
d. left ventricle
8.
Before portioning and valve formation, the common outflow for the ventricles
is the ___________.
a. truncus arteriosus
b. conus arteriosus
c. conus cordis
d. aortic sac
9.
While the right umbilical vein disappears, oxygenated blood enters the left umbilical
vein and inferior vena cava via the ____________.
a. ductus venosus
b. conus arteriosus
c. ductus arteriosus
d. portal vein
10. Shortly after the appearance of the ___________, a crescentic septum secundum
begins to form, just to the right of the septum primum.
a. ostium primum
b. atrioventricular cushions
c. ostium secundum
d. ductus arteriosus
11. The foramen ovale is an opening formed in the _____________.
a. septum primum
b. atrioventricular channel
c. ostium primum
d. septum secundum
12. The smooth-walled part of the right atrium contributed by the sinus horn is
called the _________.
a. crista terminalis
b. sinus venarum
c. aortic sac
d. conus arteriosus
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13. The arterial outflow of the heart is eventually partitioned into ____________
by fusion of two spiral conotruncal ridges.
a. right and left ventricles
b. pulmonary artery and aorta
c. right and left atria
d. superior and inferior vena cavae
14. Which of the following help form the elastic tissue in the portioning of the conotruncus.
a. lateral plate mesoderm
b. intermediate mesoderm
c. neural crest cells
d. myotubes
15. The aortic arch that contributes to the pulmonary artery is arch # _________.
a. 1
b. 3
c. 5
d. 6
16. The aortic arch that disappears is arch # ____________.
a. 1
b. 3
c. 5
d. 6
17. The reason for the left recurrent laryngeal nerve being hooked under the aorta is
because __________________.
a. the distal part of aortic arch # 5 makes the ductus arteriosus on the right
b. the distal part of aortic arch # 6 makes the ductus arteriosus on the right
c. the distal part of aortic arch # 5 makes the ductus arteriosus on the left
d. the distal part of aortic arch # 6 makes the ductus arteriosus on the left
18. The common cardinal veins drain into the ____________.
a. sinus venosus
b. umbilical veins
c. vitelline veins
d. coronary sinus
19. Which of the following does not contribute to the definitive adult inferior vena cava?
a. right subcardinal vein
b. right supracardinal vein
c. right vitelline vein
d. right umbilical vein