Basic Cardiac Embryology & Fetal Circulation
Timeline of Events
Week 3 (21 days) - heart tube forms 23 days- heart beats Week 4 - cardiac loop forms Week 7 - heart fully developed
– the 1st organ to fully develop in the fetus– resembles adult heart except for foramen
ovale
approx. 3wks approx. 4 wks approx. 7 wks
Heart tube cardiac loop forms fully developed
Primitive Cardiac Tube(bulboventricular tube) Consists of 6 parts
functioning as a tubular pump:– sinus venosus– primitive atria– primitive ventricle– bulbus cordis– conus cordis– truncus arteriosus
Cardiac Loop
As the tube grows it bends in an anterior & rightward direction forming the bulboventricular loop.
Normal looping = dextro or ‘d-looping’
Abn. looping to left = levo or ‘l-looping’
What forms what
Sinus venosus (1) Primitive atria (2)
– 1 & 2 contribute to SVC, IVC, CS, RA & LA Atrioventricular canal (3)
– large area of connection between primitive atria & ventricle
– endocardial cushion will form here; will form parts of MV & TV
More...
Primitive ventricle (4) - forms into LV Bulbus cordis: 3 parts
– Primitive RV (5)– Conus cordis (6) - will form the ventricular
outflow tracts (crista supraventricularis)– Truncus arteriosus (7) - will form the aorta
& pulmonary artery trunk
Aortic arches
6 paired sets of arches form about week 4 These develop into the adult arterial
system– 3rd set forms common & internal carotid
arteries bilaterally
– 4th develops into aortic arch
– 6th develops into right & left pulmonary arteries & ductus arteriosus
Septation(septal development) Atrial septum:
– Atrial septum begins from the atrial roof; septum primum & septum secundum separate common atria into rt & lt halves
Endocardial cushions:– divide the AV canal into 2 orifices & parts of
MV & TV; help form part of the atrial primum septum & membranous ventricular septum
Valves
AV valves (mitral & tricuspid) are formed mainly from the internal ventricular muscular wall
Aortic & pulmonic roots are formed from a separation that occurs in the truncus arteriosus. The semilunar valves are formed from small tubercles in the truncus.
Fetal circulation (prenatal)
In the adult, the lungs provide oxygen and CO2 exchange
In the fetus, the lungs are basically collapsed & fluid-filled so there is high resistance to blood flow
The placenta provides oxygen for the fetus;delivers nutrients & removes wastes
Fetal Post-natal
Right-sided pressure higher
Higher pulmonary resistance
3 shunts exist Placenta provides
oxygenated blood IVC blood is O2 rich SVC,CS is O2 poor
Lungs inflate; lowers pulm resistance/ Rt-heart pressures
Lt-heart press. rises; closes foramen ovale
Ductus arteriosus closes w/in 48-72 hrs; becomes ligamentum
Ductus venosus closes as flow ceases; becomes ligamentum
Changes at birth
When newborn begins to breathe, the baby’s body gets higher levels of O2
Pulmonary vascular resistance decreases, blood flow into the lungs increases
LAP rises; closes foramen ovale Increases O2 levels, lower vascular
resistance closes ductus arteriosus
Continued..
Clamping umbilical cord ends placental function; closes umbilical vein & ductus arteriosus
In premature infants, PFO & PDA are common
Key to diagram1. Aortic arch 11. Portal sinus
2. Ductus arteriosus 12. Portal vein
3. Pulmonary trunk 13. Umbilical vein
4. Pulmonary veins 14. Umbilical arteries
5. LA 15. Placenta
6. SVC 16. Descending aorta
7. Foramen ovale
8. RA
9. IVC
10. Ductus venosus
Sources
CV Principles: A Registry Prep, Reynolds Echocardiography, 2nd edition, Allen Textbook of Diagnostic Ultrasound, 5th
edition, Hagan-Ansert DeWitt Feigenbaum’s Echocardiography, 6th
edition.
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