Fetal circulation

Question 1

How does gas exchange occur in the fetus?

Answer 1

Fetus lungs are not functional so the placenta is where fetal blood gets oxygenated.

Question 2

How does fetal blood travel to the placenta?

Answer 2

Travels to the placenta by two umbilical arteries which originate from the internal iliac arteries. Travels back to the fetus via the umbilical vein.

Question 3

What are the 3 fetal shunts?

Answer 3

1. Ductus venosus → connects umbilical vein to inferior vena cava, allowing blood to bypass the liver.
2. Foramen ovale → connects RA to LA, allowing blood to bypass the RV and lungs.
3. Ductus arteriosus → connects pulmonary artery to aorta, allowing blood to bypass the lungs.

Question 4

What happens in the first breath after birth?

Answer 4

• The first breaths expand the alveoli, decreasing pulmonary vascular resistance
• This causes a fall in pressure in the RA and RV so left atrial pressure is greater than right atrial pressure
• This squashes the atrial septum, closing the foramen ovale which then becomes the fossa ovalis

Question 5

What is required to keep the ductus arteriosus open?

Answer 5

Prostaglandins so increased blood oxygenation causes a drop in circulating prostaglandins, causing the ductus arteriosus to close → ligamentum arteriosum.

Question 6

What stops the ductus venosus from functioning?

Answer 6

When blood stops circulating through the umbilical vein so it closes structurally a few days later to become the ligamentum venosum.