Fetal heart Review sheet

At what gestational age does the primitive heart tube form?

At 23 days of gestation.

By what day of gestation is heart development complete?

By day 43.

What forms early in embryonic cardiovascular development near the cephalic end of the fetus?

A pair of parallel heart tubes.

When does fusion of the paired heart tubes begin?

At the beginning of week three of gestation.

What is the functional significance of the primitive heart tube?

It has contractile properties that allow it to circulate blood.

From what does the contractile property of the heart tube develop?

From a specialized section of the main vascular channel.

What are the three layers of the primitive heart tube wall?

-External myocardial mantle (1-2 cell layers thick)

-Middle acellular cardiac jelly

-Internal layer of endothelial cells (1 cell layer thick)

What is the cardiac jelly, and what is unique about it at this stage?

It's the middle, acellular layer of the heart tube wall, containing no cells at this time.

What structures develop from the heart tube?

The embryonic ventricle and bulbus cordis.

What is the sinus venosus in the embryo?

A separate chamber from the fetal common atria, consisting of a central portion, transverse portion, and right and left sinus horns.

What does the left horn of the sinus venosus become in the adult heart?

The coronary sinus.

What does the right horn of the sinus venosus become in the adult heart?

It is absorbed into the right atrium as the sinus venarum and forms part of the area where the great veins open.

What gives rise to the right and left sinus valves?

Small folds on the right side of the sinus venosus.

What happens to the left sinus valve during development?

It fuses with the atrial septum.

What does the right sinus valve form?

The Eustachian valve and the Thebesian valve.

What happens to the heart tube on day 23 of embryonic development?

It bends to the right and anteriorly, forming a sigmoid shape—a process called cardiac looping.

What major vascular structures begin forming during cardiac looping?

the aortic arches form from the cephalic portion of the bulboventricular tube.

What occurs in the caudal half of the heart tube during cardiac looping?

Tissue expansion forms the early version of the adult ventricle.

Where do the primitive atria initially lie in relation to the ventricles?

Caudal to the ventricles and extrapericardially.

What is the origin of the six aortic arches in the embryo?

They develop from the aortic sac portion of the primitive heart tube.

Which aortic arches persist into adulthood

-Arch 3

-Arch 4

-Arch 6

Arch 6 becomes

Pulmonary arteries and ductus arteriosus

Arch 4 becomes

Aortic arch

Arch 3 become

Carotid arteries

What is cardiac septation?

The process of dividing the rapidly growing and expanding heart tube into the four chambers of the adult heart.

Cardiac septation occurs simultaneously with the development of which three structures?

-primitive ventricles

-conus cordis

-truncus arteriosus.

What are the three mechanisms of cardiac septation?

-Passive septation

-Active septation

-Combination of passive and active septation

What is passive septation?

Fusing of growing and expanding wall segments of the primitive heart chambers.

What is active septation?

Growth of cardiac mesenchyme (endocardial cushion tissue) derived from cardiac jelly that forms parts of the heart septa.

What happens in combined passive and active septation?

Septation begins passively but is completed by endocardial cushion tissue that grows and fuses at the borders.

How many septa are formed during cardiac septation?

Seven septa.

Of the seven septa formed in cardiac septation, how many are formed by each method?

-3 by passive septation

-3 by active septation

-1 by a combination of both

How are heart valves initially formed?

By infolding and growth of the muscular ventricular wall through diverticulation.

How is valve formation completed after muscular infolding?

Through expansion and fusion of endocardial tissue.

How do atrioventricular valves change from early development to adulthood?

They are thick and fleshy early on but become thin fibrous cusps in the adult heart.

Where does highly oxygenated blood in the fetus originate?

From the placenta, traveling through the umbilical vein.

What are the two pathways for blood entering the fetal liver?

-Half passes through the liver

-Half bypasses via the ductus venosus into the IVC

What happens to blood entering the right atrium from the IVC in the fetus?

It mixes with desaturated blood from the SVC and is directed through the foramen ovale into the left atrium.

Where does the small portion of blood that crosses the tricuspid valve go in fetal circulation?

It enters the right ventricle, then the pulmonary artery, and is mostly shunted to the descending aorta via the ductus arteriosus.

What is the ductus arteriosus?

A fetal vessel connecting the pulmonary artery to the aorta, allowing blood to bypass the lungs.

What are the three fetal circulatory shunts?

-Ductus venosus

-Foramen ovale

-Ductus arteriosus

What happens to the ductus venosus after birth?

It constricts, forcing all blood to pass through the hepatic sinusoids of the liver.

What causes the foramen ovale to close after birth?

-Loss of placental circulation drops right atrial pressure

-First breath decreases pulmonary resistance and increases left atrial pressure, pushing the flap of the foramen ovale closed

What change occurs in pulmonary vascular resistance after birth?

It dramatically decreases, increasing pulmonary blood flow.

What triggers the constriction of the ductus arteriosus at birth?

The initial inflation of the lungs releases bradykinins, which cause the ductus arteriosus to constrict.

Is the transition from fetal to adult circulation immediate?

No, the change is gradual and can take several weeks.

During the transition to adult circulation, what happens to flow through the fetal shunts?

There is still some flow through the ductus venosus, foramen ovale, and ductus arteriosus during the early postnatal period.

At what gestational age does the development of the cardiac tube begin?

Around 3 weeks gestational age.

What forms the primitive heart during the cardiac tube stage?

A pair of endothelial tubes that fuse to form a primitive cardiac tube.

How many parts can the primitive cardiac tube be described as having?

Six parts, each giving rise to different structures of the adult heart.

When does cardiac looping begin?

week 4 of gestation

Why does the heart tube begin to loop during development?

Because it lengthens but is confined within the pericardial cavity, causing it to bend into an S-shape.

What important structural relationship is established by cardiac looping?

The atria and ventricles move into their final relative positions.

When does the heart begin to beat spontaneously?

Around the time of cardiac looping, at ~4 weeks gestation.

In normal development, which direction does the heart loop?

Ventrally and to the right (called dextro-position).

What condition can result if the heart loops to the left (levo-position)?

Congenitally corrected transposition of the great arteries.

What does the primitive ventricle become after looping?

The left ventricle.

What does the bulbus cordis become after looping?

The right ventricle.

What does the primitive atrium become?

right and left atria, following septation.

What happens to the sinus venosus during looping?

It is incorporated into the right atrium.

When does cardiac septation occur?

Between 33-37 days of gestational age.

What does septation accomplish in heart development?

It divides the common atria and ventricles into right and left sides, forming the AV canal.

What fuses during septation to separate the AV canal?

The endocardial cushions.

What valves are formed during septation?

The mitral and tricuspid (atrioventricular valves).

What is the common AV canal?

A large, single chamber where atria and ventricles communicate freely before septation.

What are endocardial cushions?

Localized clumps of tissue found on the anterior (ventral) and posterior (dorsal) walls of the common AV canal.

What happens when the endocardial cushions fuse?

-Forms two AV canals (right and left)

-Divides AV valve openings

-Contributes to interatrial and membranous interventricular septa

What septum does the upward growth of the endocardial cushions help form?

The interatrial septum (septum primum).

What septum does the downward growth of endocardial cushions help form?

The membranous interventricular septum.

When does aortic arch formation occur?

4th week

What is the aortic arch derived from?

It is one of six paired aortic arches that form from the aortic sac.

What is an Atrial Septal Defect (ASD)?

An abnormal opening in the interatrial septum causing left-to-right shunting of blood.

What happens to pulmonary pressure in ASD?

It becomes greater than systemic pressure due to left-to-right shunt.

What are the three types of ASDs?

-Ostium secundum

-Ostium primum

-Sinus venosus

Describe Ostium Secundum ASD:

-Located in the mid-portion of the interatrial septum

-75% more common in women

Describe Ostium Primum ASD:

-Located at the inferior portion of the IAS

-Often associated with a cleft mitral valve

Describe Sinus Venosus ASD:

-Located posteriorly and superiorly in the IAS

-Near the entrance of the SVC

What is Eisenmenger’s complex in ASD?

A condition where right-sided pressure increases, causing a reversal of the shunt direction (right-to-left).

What are the types of VSDs?

-Perimembranous (most common in adults)

-Trabecular (muscular)

-Inlet (AV canal, posterior)

-Outlet (subpulmonic or supracristal)

What is coarctation of the aorta?

A narrowing of the aorta, usually near the origin of the left subclavian artery.

What is the hemodynamic effect of aortic coarctation?

Increased pressure proximal to the narrowing and decreased pressure distal to it.

What are the possible levels of congenital aortic stenosis?

-Valvular

-Subvalvular

-Supravalvular

What is the most common form of pulmonic stenosis?

Valvular pulmonary stenosis, usually with a dome-shaped, narrowed opening.

What is a dysplastic pulmonary valve?

A valve with thickened, fused leaflets in pulmonary stenosis.

What is a Patent Foramen Ovale (PFO)?

A persistent opening between the atria (normal in fetus), present in ~20% of adults.

Does a PFO always cause chamber enlargement?

No, L/R shunting may occur without right-sided enlargement.

What is a Patent Ductus Arteriosus (PDA)?

A persistent fetal vessel that connects the pulmonary artery to the aorta after birth.

What is a Partial AV Canal Defect?

A congenital defect involving an ostium primum ASD and a cleft mitral valve.

What is Ebstein’s Anomaly?

A congenital malformation where one or more tricuspid valve leaflets are displaced downward into the right ventricle.

What anomalies are associated with Ebstein’s Anomaly?

-ASD (~75%, often R-L shunt)

-PFO (patent foramen ovale)

-VSD

-Pulmonic stenosis

What are the four defects in Tetralogy of Fallot?

-Large VSD

-Pulmonary stenosis

-Overriding aorta

-Right ventricular hypertrophy (RVH)

What is Hypoplastic Left Heart Syndrome?

A left-sided congenital defect where structures like the LV, mitral valve, aortic valve, and aorta are underdeveloped.