Week 6 Embryology of Fetal Circulation

Basic Embryology of the Heart

  • Seventh week of development
  • Most sensitive phase: 3.5 to 6.5 weeks
  • Prior to week three:
    • Heart consists of a pair of tubes on either side of the midline.
    • Cephalic end (middle part of the embryo)
    • Two layers: inner thinner layer (endocardium) and outer thicker layer (myocardium).
  • 21 to 23 days: Heart tubes forced together to the midline, fusing into a single endocardial heart tube.
    • Connects with the developing aortic arch system at the cephalic end and the sinus venosus at the caudal end.
    • Divisions (caudally):
      1. Sinus Venosus: Formed by the union of three pairs of veins (common, vitelline, umbilical).
        • Forms superior vena cava, inferior vena cava, pulmonary veins, coronary sinus, oblique vein of the left ventricle, and posterior portions of the right and left atria.
      2. Primitive Atrium: Divides into right and left atria upon development of the interatrial septum.
      3. Atrial Ventricular Sulcus: Divides the primitive atrium and primitive ventricle.
      4. Primitive Ventricle:
        • Originates superior to the primitive atrium but moves inferior upon normal looping.
        • Gives rise to trabeculae expansion (inferior growth) forming the apical portion of the left ventricle.
        • Contributes to the inlet portion of the right ventricle.
      5. Interventricular Sulcus: Divides the primitive ventricle and the bulbous cordis.
      6. Bulbous Cordis: Divided into three parts.
        • Proximal: Gives rise to trabecular expansion forming the apical portion of the right ventricle.
        • Mid (Conus Cordis): Connects the right ventricle with the truncus arteriosus; forms right and left ventricular outflow tracts.
        • Distal (Truncus Arteriosus): Forms the aorta and main pulmonary artery.
      7. Aortic Sac: Gives rise to five pairs of aortic arches.
        • Left fourth arch forms definitive aortic arch.
        • Distal left sixth arch forms ductus arteriosus.
        • Other arches/portions form proximal parts of right/left pulmonary arteries, innominate artery, right subclavian artery, and common carotid arteries.
        • Some aortic arches disappear by birth.

Heart Development Details

  • Bilateral heart tube fuses into a single tube.
  • Muscular contractions begin (heart starts beating).
  • Initial heart rate: 60-70 bpm; by birth: 130-150 bpm.
  • Cardiovascular system: First system to function in the embryo.
  • Heart grows rapidly, folding into an S-shaped loop (d-looping). Usually folds to the right.
  • Bulboventricular loop formed; heart finds correct anatomical position in the chest cavity.
  • Week Four:
    • Septum primum appears from the dorsal wall of the primitive atrium, growing towards the endocardial cushions.
    • Ostium primum remains.
    • Sinus venosus and primitive atrium communicate via sinoatrial orifice.
    • Sinoatrial orifice shifts to the right, communicating solely with the right atrium.
    • Endocardial cushions appear, serving as primitive valves within the atrioventricular canal.
    • Atrioventricular canals provide communication between primitive atrium and ventricle. Initially one common orifice that separates into left and right sides that will become the mitral and tricuspid valves.
    • Ventricular septum appears as small ridges on the floor of the primitive ventricle; ventricle begins to expand.
    • Only one great vessel leaving the heart: truncus arteriosus, which later forms the aorta and main pulmonary artery.
  • Week Five:
    • Septum primum reaches endocardial cushions, closing ostium primum.
    • Perforations develop within septum primum, forming ostium secundum.
    • Septum secundum develops from the superior portion of the atrium to the right of the septum primum.
    • Sinoatrial orifice shifts entirely to the right, communicating solely with the right atrium.
    • Right and left venous valves flank the sinoatrial orifice. The left venous valve and septum spurium fuse with the septum secundum as it grows.
    • The orifice of the coronary sinus is situated at the base of the sinoatrial orifice.
    • Right and left ventricles originate as trabecular expansion grows inferiorly from the bulbous cordis and primitive ventricle.
    • Ventricles separated externally by the interventricular sulcus and internally by the bulboventricular flange.
    • Trabecular expansion from the bulbous cordis forms the apical portion of the right ventricle.
    • Conus (midportion) of the bulbous cordis contributes to right and left ventricular outflow tracts.
    • Trabecular expansion from the primitive ventricle forms the apical portion of the left ventricle.
    • A portion of the primitive ventricle contributes to the inlet portion of the right ventricle.
  • Travicular Intraventricular Septum:
    • Forms as ventricles grow and expand from the bulbous cordis/primitive ventricle until late seventh week.
    • Stops growing when ventricular walls stop expanding.
    • In adult heart, this portion is known as the muscular septum.
    • The primitive atrium communicates directly with the left ventricle via the atrioventricular canal and the primitive ventricle.
    • The primitive atrium cannot directly communicate with the right ventricle.
    • The atrioventricular canal shifts to the right to communicate with the right ventricle.
    • Left ventricle communicates with right ventricle via primary interventricular foramen or the ventricular bulbar foramen; secondary communication is in the aortic annulus.
    • Right intraventricular septum will take its the lateral ventricle. The canal, also known as the infundibular or outlet.
    • The intraventricular septum is located at the level of the ventricular outflow tracts.
  • Week Six:
    • Septum secundum stops growing, forming the foramen ovale (shunt between right and left atria).
    • Superior and inferior endocardial cushions fuse.
    • Atrioventricular valve cusps form.
    • right atrium communicates with the right ventricle, and the left atrium communicates with the left ventricle.
  • Week Seven:
    • Heart is in its final stage of development.
    • The remnant of the septum primum acts as a flap or valve at the level of the foramen ovale, controlling blood flow between the atria.
    • At birth, the infant takes its first breath, increasing left atrial pressure and forcing the septum primum against the septum secundum, closing the foramen ovale.
    • Septum primum and septum secundum together form the interatrial septum.
    • The secondary intraventricular foramen is obliterated by the membranous septum, leaving distinct right and left ventricles.
    • Heart development is largely complete; fetal circulation is in place.

Fetal Circulation

  • Adult vs. Fetal Circulation:
    • Adult: Lungs responsible for O2 and CO2 exchange; kidneys, liver, gallbladder remove waste.
    • Fetus: Placenta delivers oxygen and nutrients since lungs are filled with fluid (high pulmonary resistance).
    • Placenta also removes waste.
    • Adult: Left heart pressure > right heart pressure.
    • Fetus: Right heart pressure > left heart pressure.
    • Pressure difference combined with pulmonary resistance forces blood to bypass lungs via shunts.
  • Shunts:
    • Foramen Ovale: From right atrium to left atrium.
    • Ductus Arteriosus: From pulmonary artery to aorta.
    • Adult: Left heart has high oxygen saturation (98%); right heart has low oxygen saturation (75%).
  • Fetal Circulation Pathway:
    • Begins and ends at the placenta.
    • Fetus and mother exchange O2, CO2 ,nutrients, and waste in the placenta (blood not mixed).
    • Enriched blood and nutrients travel from placenta via single umbilical vein (80% O_2 sat).
      • The umbilical vein is the ONLY vessel carrying enriched blood.
      1. Small amount enters portal system to mix with liver's depleted blood.
      2. Majority of umbilical vein blood goes into ductus venosus, connecting directly to the inferior vena cava (bypassing the liver).
      3. In the inferior vena cava, enriched blood from ductus venosus mixes with depleted blood returning from the lower body.
    • From inferior vena cava, blood passes through eustachian valve into right atrium.
      1. Small amount of enriched blood remains in the right atrium It mixes with depleted blood returning from the head and upper body via the superior vena cava, and it's gonna continue to the right ventricle and pulmonary artery.
      2. Majority Enriched blood directed across right atrium through foraminal veil and into left atrium, bypassing lungs (first shunt).
    • Blood in left atrium mixes with small amount of blood returning from lungs and enters left ventricle and aorta.
      • The myocardium in the brain can receive enriched blood from the aorta by way of the coronary arteries and the carotid arteries.
    • Oxygen-depleted blood from upper body and brain returns via superior vena cava to right atrium, right ventricle, and pulmonary artery.
    • High pulmonary resistance causes majority of blood to shunt from pulmonary artery to aorta via ductus arteriosus (second shunt).
      • Once in the aorta, depleted blood mixes with enriched blood from proximal aorta.
    • Abdominal aorta branches into right and left common iliac arteries; these branch into internal and external iliac arteries.
    • Internal iliac arteries give rise to two umbilical arteries, returning depleted blood (58% O_2 sat) to placenta.
    • Small amount of blood that doesn't shunt remains in pulmonary artery, travels through lungs and returns to left atrium via four pulmonary veins.

Changes at Birth

  1. Placental blood flow interrupted: O2 level decreases, CO2 level increases.
  2. Newborn takes a breath: Lungs fill with air; body experiences higher O_2 levels decreasing vascular resistance and increasing pulmonary blood flow.
  3. Increased pulmonary blood flow increases blood returning to the left atrium, increasing left atrial pressure and forcing the fossa ovalis to close.
  4. The Ductus arteriosus closes almost immediately after birth (ligamentum arteriosum), due to muscular contractions.
  5. Umbilical arteries contract shortly after birth due to increased O_2 levels, obliterating in 2-3 months.
  6. Umbilical vein and ductus venosus close (ligamentum venosum), allowing time for any remaining blood in the placenta to reach the newborn before the umbilical cord is cut.