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Cardiovascular Development
• The first three weeks
• By the beginning of the third week, blood vessel
formation begins in the tissue surrounding the
yolk sac.
• These primitive vessels are at first not
connected to each other, but do start to make
primitive blood cells.
• In the third week the heart and large vessels
form in the cardiogenic area from mesoderm.
• The first step in formation of the cardiovascular
system is the development of two simple tubes
lying on either side of the midline in the
mesoderm.
• These tubes are connected to:
• umbilical arteries connected to vessels in
the chorion (primitive placenta):
• vitelline arteries to the yolk sac;
• dorsal intersegmental arteries to the body
of the embryo.
• Umbilical veins, vitelline veins and cardinal
veins return the blood to the circulation.
• At 21 days the two longitudinal tubes (the
dorsal aortae) fuse at their cranial ends to
form the primitive heart.
• Heart beats can be detected by ultrasound
by the fifth week.
• At 20 days the circulation is
primitive but the three areas to
be connected are present –
• the chorion (early placenta),
• the yolk sac (vitelline arteries)
• and the body (intersegmental
arteries).
• Development of the heart
• Fusion of the primitive heart tubes into a
single atrioventricular tube is the initial
event in development of the heart.
• Mesoderm around the endothelial tube
differentiates into myocardium.
• As the head fold caries the developing
heart caudally, ventral to the foregut and
caudal to the oropharyngeal membrane,
the heart tube develops dilatations forming
the truncus arteriosus, bulbus cordis,
ventricle, atrium, and sinus venosus.
• The truncus arteriosus connects the heart to the
future arterial system
• and the sinus venosus receives blood from the
umbilical, vitelline and common cardinal veins
(i.e. from the chorion or placenta, from the yolk
sac, and from the embryo).
• Growth of the heart tube is uneven, but it is fixed
to adjacent tissue at its two ends - to the
pharyngeal arches cranially and to the primitive
diaphragm caudally.
• Accelerated growth of the bulbus cordis and
ventricle causes the heart tube to bend and
invaginate the pericardium.
• The next steps involve partitioning of the tube
into atria and ventricles.
• Partitioning of the heart
• During the fourth week the dorsal and ventral
walls bulge into the lumen of the tube to form an
atrial and a ventricular chamber, connected by
left and right atrioventricular canals.
• The primitive atrium is divided into right and left
atria by the growth of septa.
• The septum primum grows down towards the
endocardial cushions.
• As the septum grows down the space below it is
termed the foramen primum.
• Eventually the septum primum reaches and
fuses with the endocardial cushions closing the
foramen primum.
• As the septum primum ends its
descent an aperture appears in its
substance, the foramen
secundum.
• At this stage then the septum
primum separates the atrial
chamber into right and left,
although there is a hole (the
foramen secundum) in the
septum primum, allowing
communication between the atria.
• During the fifth week a second
septum, the septum secundum grows
down on the right side of the septum
primum, to eventually overlap the
hole in the septum primum.
• The septum secundum is crescent
shaped and does not entirely reach
the floor of the right atrium.
• The oval shaped gap,allowing blood
topass through the interatrial septum,
is the foramen ovale.
• While the atrium is undergoing partitioning
the same process is occurring in the
ventricle.
• A muscular ridge grows up from the floor
of the ventricle towards the atrioventricular
cushions.
• As the septum grows upwards itn leaves a
communication between the two
ventricles, the interventricular foramen.
• By the seventh week the interventricular
foramen is closed by the fusion of tissues
from the interventricular septum and the
endocardial cushions.
• During the fifth week there is
partitioning of the truncus
arteriosus and bulbus cordis.
• The spiral septum developing in
this outlet from the heart
eventually fused with the
endocardial cushions so that the
output from the right ventricle
enters the pulmonary trunk, and
the output from the left ventricle
enters the aorta.
• The sinus venosus of the primitive heart
opened into the common atrium.
• As development proceeds the opening of
the sinus venosus moves to the right
atrium.
• As the sinus venosus moves to the right
the venous drainage from the rest of the
body, the head, the vitelline and umbilical
veins also move to the right.
• A small part of the sinus venosus still lies
behind the left side of the heart, to become
the coronary sinus.
• The fetal circulation
• In the fetus the four-chambered heart
circulates blood through the fetal body,
through the placenta and through the
vessels supplying the gut (previously the
vitelline vessels)
• The blood arriving at the right side of the
heart through the superior and inferior
venae cavae has come from the fetus
(body and head), from the digestive
system and liver, and from the umbilical
vein.
• The blood in the umbilical vein is highly
• The blood mixes in the right
atrium and is directed towards the
interatrial septum where it passes
through the foramen ovale into
the left atrium.
• From the left atrium it passes into
the left ventricle and so to the
systemic circulation through the
aorta.
• Any blood which passes into
the right ventricle leaves that
chamber though the
pulmonary trunk to pass to the
aorta through the ductus
arteriosus, but also to a small
extent to the lungs.
• The aorta thus distributes blood to the
head, the body including the gut, and
to the placenta through the paired
umbilical arteries.
• At the placenta the blood is
oxygenated and collected in the
single umbilical vein.
• The umbilical vein travels to the liver
where a connection, the ductus
venosus, shunts most of the blood
into the inferior vena cava.
• This circulation ensures that
the head and upper limbs
receive oxygenated blood, that
the body receives fairly well
oxygenated blood with some
addition of venous blood from
the head and lungs.
• Since at this stage the lungs are
not inflated, the resistance in the
pulmonary circulation is high and
the blood is diverted back into the
systemic circulation through the
ductus arteriosus.
• The ductus venosus diverts blood
from the umbilical vein bypassing
the liver, directly into the IVC.
• Changes at birth
• At birth the lungs will inflate and the
placenta will become disconnected from
the source of oxygenation.
• With the first breath the resistance in the
pulmonary circulation falls as the lungs
inflate.
• Blood in the right ventricle is able to pass
through the pulmonary circulation where
oxygenation occurs.
• Oxygenated blood from the lungs returns
to the left atrium.
• Cutting and tying the umbilical cord closes
both the umbilical arteries and the
umbilical vein.
• Other significant changes in the circulation
occur over the next hours and days.
• The ductus arteriosus starts to constrict
and is usually permanently closed by three
months after birth.
• As pressure in the left atrium rises, the
septum primum is forced against the
septum secundum, closing the foramen
ovale.
• Eventually the two septa fuse to
permanently separate the two atria.
• Absence of flow in the umbilical vein
causes it to become ligamentous - the
ligamentum teres, although a narrow
channel remains.
• The ductus venosus becomes the
ligamentum venosum and the blood
from the portal vein must pass
through the liver before reaching the
IVC.