Development of the
Circulatory System – Part II
Learning Objectives:
Compare and contrast the flow of blood through a fetal and adult heart.
Identify which parts of the heart are derived from the primary and secondary
heart fields.
Describe how embryonic folding creates the primitive heart tube.
Explain how the heart's shape changes during cardiac looping.
Describe how the atrial septum and interventricular septum are formed.
Explain the function of the three fetal circulatory shunts.
Explain how blood flow in an infant changes at birth. What happens to the
three shunts?
Intraembryonic circulatory arc:
• Flow of blood away from heart:
• Heart – aortic sac – aortic arches – dorsal aorta
• A system of cardinal veins brings the blood back to the heart
• Cranial cardinal vein & caudal cardinal vein – common cardinal vein – heart
Development of the heart:
After leaving the primitive streak, precardiac cells move anteriorly and form the
primary heart field (cardiac crescent) which is made from cardiogenic mesoderm
(splanchnic mesoderm)
A second heart field is located medially to the cardiac crescent
As the heart continues to develop, a cardiogenic plate forms from the cardiogenic
mesoderm just rostral to the buccopharyngeal membrane
Cells of the primary heart field give rise to:
• Ventricles
• Left atrium
• Some of the right atrium
• Some of the outflow tract
Cells of the secondary heart field give rise to:
• Most of the outflow tract
• Most of the right atrium
• The splanchnic and somatic mesoderm split
• The space between is called the pericardial coelom – precursor to the
pericardial cavity
• The splanchnic mesoderm of the precardiac region thickens to form the
myocardial primordium
• The endocardial primordia forms as a tube between the myocardial primordium
and the endoderm of the yolk sac/primitive gut
• Lateral/ventral folding events of the early embryo bring the primordia together
where they fuse at the midline (ventral to the gut) to form the primitive heart tube
• Two separate lumen become the single lumen of the heart
• The primitive single tubular heart consists of:
• Endocardial lining
• Cardiac jelly
• Myocardium
• The tubular heart is located in the pericardial coelom
• Formation of the tubular heart has occurred by the end of the third week
Vitelline
veins
Aortic sac
Body
Right atrium
Right ventricle
Lungs
Left atrium
Left ventricle
Review of adult circulation
Cardiac looping:
Around day 23, the primitive heart
folds and loops to establish the
future heart chambers in the
correct spatial locations
The heart is the first asymmetrical
structure to develop in the embryo
The initially straight heart tube
begins to take on an S shape
• The inflow tract (atrium)
becomes positioned dorsal to
the outflow tract (conotruncus)
As the heart continues to grow, the
atrium can be seen bulging out on
either side of the heart
Video
Atrioventricular partitioning begins when endocardial cushions begin to form
• Endocardial cushions – thickenings on the dorsal and ventral sides of the
heart at the junction of the atrium and ventricle
The cushions will eventually grow into the atrioventricular canal and meet,
separating it into left and right channels
The right atrioventricular canal will develop into the tricuspid valve
The left atrioventricular canal will develop into the mitral (bicuspid) valve
1. Left atrioventricular canal
2. Right atrioventricular canal
1. Lateral endocardial cushion
2. Ventral endocardial cushion
3. Dorsal endocardial cushion
4. Left atrioventricular canal
5. Right atrioventricular canal
Partitioning of the atria begins at the
same time as atrioventricular
partitioning
• It begins with the downward
growth of the interatrial septum
primum, located between the
atrial chambers
• The interatrial septum primum
grows toward the atrioventricular
canal and merges with the
endocardial cushions
• A large opening called the
foramen primum allows blood to
pass from the right to left side
• When the septum primum meets
the endocardial cushions, the
foramen primum will be closed
• Perforations in the septum
primum become the foramen
secundum
• At weeks 5-6, a second atrial
septum, the septum secundum
descends to the right of the
septum primum
• The septum secundum will cover
and close the foramen secundum
• A oval-shaped passageway called
the foramen ovale will be formed
between the septum secundum
and septum primum
• The foramen ovale allows blood to
shunt right to left until birth,
bypassing the lungs
The sinus venosus shifts
completely to the right atrium
• Remember, the common
cardinal vein, umbilical vein,
and vitelline vein all empty into
the sinus venosus
Partitioning of the ventricle begins
with the formation of the
interventricular septum
• The interventricular septum
grows from the apex of the
ventricular loop and eventually
merges with the endocardial
cushions
• The interventricular septum
has two parts:
• Muscular septum
• Membranous septum
• The membranous septum is
derived from neural crest cells
and is a common defect site
In the tubular heart, the outflow
tract is a single channel
With the formation of the
interventricular septum, the outflow
tract separates into two channels:
1. Aortic outlet – connects to the
left ventricle. Becomes aorta
2. Pulmonary outlet – connects to
the right ventricle. Becomes
pulmonary artery.
The aorticopulmonary septum is
a spiraling ridge of tissue that
divides the outflow tract.
Semilunar valves (aortic and pulmonary valves) form where the outflow tract
meets the ventricles
Body
Right atrium
Right ventricle
Lungs
Left atrium
Left ventricle
Adult circulation in the heart
Sinus venosus
Atrium
Ventricle
Conotruncus
Aortic sac
Aortic arches
Dorsal aorta
Descending aorta
Dorsal aorta
Vitelline arteries
Yolk sac
Vitelline veins
Body
Cranial cardinal vein
Caudal cardinal vein
Common cardinal vein
Umbilical arteries
Placenta
Umbilical veins
(Ductus venosus) Fetal circulation
Three shunts in fetal circulation are needed to supply highly oxygenated blood to
the body and developing brain:
1. Foramen ovale
• Opening between right and left atria
• Shunts highly oxygenated blood from right atria to left atria
• Blood moves from LA to LV, aorta, body
2. Ductus arteriosus
• Temporary blood vessel connecting pulmonary artery and aorta
• Derived from left 6th aortic arch
• Allows blood leaving right ventricle to bypass lungs
• Fetal lungs are not fully developed and cannot handle the full amount of
blood entering the pulmonary artery
3. Ductus venosus
• Connects umbilical vein to sinus venosus
• Allows oxygen-rich blood returning from placenta to bypass liver
• Liver is a dense capillary bed that would deoxygenate blood as it slowly
passed through
Oxygenated blood from the placenta is
carried by the umbilical vein past the
liver via the ductus venosus. It empties
into the sinus venosus, which in turn
empties into the right atrium
The right atrium would normally send
the blood to the lungs, but instead the
foramen ovale allows blood to be
shunted directly to the left atrium
Blood then travels through the left
atrium to the left ventricle and out to the
body of the embryo
Enough blood passes into the
pulmonary artery to supply the lungs
with the oxygen they need
Only 12% of the right ventricle output
goes to the lungs, rest travels to ductus
arteriosus
Fetal circulation summary
The embryo must prepare for the moment when oxygenating the blood must be
done using the lungs instead of the placenta
When the umbilical cord is cut, all blood flow via the umbilical vein stops
The baby takes a few breaths, and the lungs expand enough to hold a larger
amount of blood
More blood starts being directed to the lungs, and this combined with the lack of
blood flow from the umbilical vein causes the blood pressure of the left atrium to
increase with respect to the right atrium
• This increase in blood pressure causes the foramen ovale collapse on itself,
and all blood from the right atrium will begin to enter the right ventricle
Ductus venosus begins closing when umbilical veins are occluded due to loss of
blood flow. Fully closed by 1 week.
Ductus arteriosus closes quickly after birth. Wall of the vessel constricts in
response to oxygen, prostaglandin levels.
For each shunt, functional closure is rapid. Anatomical closure takes longer.
Circulation after birth
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