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