LM 15.12: Fetal-Neonatal Transition

Question 1

how does blood circulate through a fetus?

Answer 1

1. placenta
2. umbilical vein
3. liver/portal circulation

ductus venous actually takes most of the blood to bypass blood to the inferior vena cava and straight to the fetal heart

once it gets to the RA it mixed with deoxygenated blood and it can continue to the RV/lungs orrrr it’s more likely to go through the foramen oval to the LA and out the aorta since RV/lung pressure is super high – this happens because pulmonary pressure is super high during gestation due to hypoxic vasoconstriction since the lungs are filled with amniotic fluid

of the blood that does go to the RV and pulmonary arteries, the ductus arteriosus is another bypass that this blood can take to bypass the lungs and go to the aorta!

as you go through the aorta and pick up deoxygenated blood and reach the iliac vessels, those join to make the umbilical arteries which head to the placenta to get oxygenation and this happens easily because the placenta has super low resistance!

Question 2

what are the fetal circulatory adaptations that allow fetal circulation to work during gestation?

Answer 2

1. umbilical vein
2. ductus venosus
3. foramen ovale
4. ductus arteriosus
5. low resistance within placenta

Question 3

how does the fetal circulatory system change at birth to adapt for life outside the uterus?

Answer 3

when the temperature falls around the umbilical cord at birth, the wharton’s jelly contracts and squeezes the vessels inside which increases resistance in the cord so blood flow via the umbilical cord stops – and then on top of that, we also clamp the umbilical cord too – both of these create a very high resistance “placenta” so blood no longer goes to the placenta!

also when the baby takes its first breath, amniotic fluid gets pushed out of the alveoli and oxygen fills instead so now the lungs are low resistance because the hypoxic vasoconstriction is no longer occurring – so not blood is getting oxygenated in the lungs!

due to this greatly decreased pressure in the RA and RV, the pressure gradient between LA and RA gets reversed and the LA pressure becomes much higher relative, pushing the septum premium and septum secundum together which closes the foramen ovale! this happens within minutes of clamping the cord

the ductus arteriosus between the pulmonary artery and aorta – DA has smooth muscles and once it senses increased oxygen levels it begins to vasoconstriction and obliterate itself; also loss of prostaglandin levels cause it to close too – blood more easily flows through pulmonary artery because the lungs have decreased resistance and there’s a huge pressure gradient in the aorta so it’s easier for blood to go through pulmonary artery

Question 4

what 2 factors cause the ductus arteriosus to close?

Answer 4

1. decreased prostaglandin levels (placenta makes lots of prostaglandins)
2. increased oxygen levels cause vasoconstriction and obliteration (DA is oxygen sensitive)

Question 5

what happens to the ductus venous after birth?

Answer 5

it’s obliterated because there is no longer an umbilical vein that is returning oxygenated blood from the placenta to fetus

Question 6

how does peripheral vascular resistance change at birth?

Answer 6

it decreases to allow better blood flow to the lower extremities!