CV development

Question 1

Early circulation

Answer 1

• Inflow of the heart is the sinus venosus, which becomes the sinus venarum (on the right side) in the adult heart
• 3 inflow sources: embryo, yolk sac, and placenta
• Placenta is only source of oxygenated blood, which is brought to fetus by umbilical vein

Question 2

Looping

Answer 2

• 3rd week-6th week: converts early linear heart tube into a structure similar to mature heart
• The atrial chamber is brought superior and posterior to the ventricular chamber

Question 3

Atrial maturation

Answer 3

• The single atrial chamber is connected to the ventricular chamber thru the AV canal
• Endocardial cushions grow within the AV canal and will become the tricuspid and mitral valves (canal is divided into right and left side
• The sinus venosus on the left side will become the coronary sinus

Question 4

Atrial septation and foramen ovale 1

Answer 4

• Atrial septum is formed from 2 things: septum primum, which extends from the roof of the atria, and septum secundum, which forms from the floor of the atria
• Neither of these growths completely fuse with the opposite side, meaning there are 2 holes (one in each growth) but these holes are off-set
• The gap is foramen ovale

Question 5

Atrial septation and foramen ovale 2

Answer 5

• Septum secundum is thicker and rigid, and septum primum is really just a flap lying over the gap in septum secundum
• Thus, septum primum serves as a one-way valve over foramen ovale for blood to flow from RA to LA, but prevents flow in the reverse direction

Question 6

Ventricular maturation

Answer 6

• The inter ventricular septum (IVS) forms btwn the left and right AV canals
• Complete separation of the ventricles relies on fusion of the septum with the endocardial cushion, and the tissue that grows down from the outflow tract septum

Question 7

Outflow tract septation

Answer 7

• Two opposing ridges (cushions) form longitudinally on the inner surface of the outflow tract
• They grow towards each other and ultimately fuse to form a septum separating the R and L outflow tracts
• The two outflow tracts assume a spiral orientation around each other
• The bottoms of the outflow tract septum continues to grow downwards and fuses w/ the muscular IVS to fully divide the RV from LV

Question 8

Aortic arch arteries (pharyngeal arch derivatives)

Answer 8

• Left and right 3rd arches become common carotid arteries
• Left 4th arch becomes arch of the aorta
• Right 4th arch becomes the brachiocephalic artery
• Left 6th arch becomes the left PA and ductus arteriosus
• Right 6th arch forms the R PA (distal portion regresses)

Question 9

Position of recurrent laryngeal nerves

Answer 9

• Since the distal portion of the left 6th arch becomes ductus arteriosus but the distal R 6th arch regresses, the recurrent laryngeal nerves (from the 6th arch) will be in different places on the two sides
• On the R side the nerve is near under the subcalvian because the distal portion is gone, allowing the nerve to migrate superiorly
• But on the left side the distal portion becomes the ductus arteriosus, thus the nerve is trapped under the aorta and cannot move superiorly

Question 10

Fetal shunts

Answer 10

• Ducutus venosus: oxygenated blood from umbilical vein bypasses fetal liver via ductus venosus to reach the IVC
• Becomes ligamentum venosum
• Foramen ovale: opening in the atrial septum to let oxygenated blood from IVC to reach the LA so it can bypass the lungs and go straight to systemic circulation
• Foramen ovale becomes fossa ovalis
• Ductus arteriosus: shunts blood from main PA to desc Ao so the blood mostly bypasses the lungs
• Becomes ligamentum arteriosum
• Umbilical vein becomes ligamentum teres

Question 11

Changes at birth

Answer 11

• Before birth the pressure in RA is higher than pressure in LA so blood can stream thru foramen ovale
• Resistance in the lungs before birth is very high, meaning most blood favors bypassing the lungs, either thru foramen ovale or thru ductus arteriosus
• At birth when the infant breathes the lungs expand and the resistance in the lungs decreases, leading to a great increase in pulmonary blood flow
• This process leads to a rise in LA pressure (LAP now greater than RAP), forcing septum primum to close over septum secundum and closing foramen ovale
• Ductus arterioles now is not needed, and closes due to increase in pO2 and decrease in PGE1