Fetal circulation

Question 1

What is the ductus venosus and how much blood flows through it on average?

Answer 1

It is venous shunt that allows oxygenated blood from the umbilical to bypass the liver and enter the inferior vena cava. At least half (70 - 130 mL/min/kg after 30 weeks) of estimated umbilical blood flow

Question 2

What is the foramen ovale?

Answer 2

An interatrial septum shunt allowing blood to flow from the right to left atrium

Question 3

What is the ductus arteriosus?

Answer 3

A connection between the left pulmonary artery and aorta that shunts blood away from the lungs and into the fetal and placental circulations.

Question 4

What percentage of biventricular (total) cardiac output is contributed by the right ventricle in the fetus?

Answer 4

60%

Question 5

What percentage of biventricular (total) cardiac output goes to the placenta?

Answer 5

40%

Question 6

What percentage of biventricular (total) cardiac output goes to the fetal body?

Answer 6

25%

Question 7

What percentage of biventricular (total) cardiac output goes to the fetal brain?

Answer 7

13%

Question 8

What percentage of biventricular (total) cardiac output goes to the fetal lungs?

Answer 8

7%

Question 9

Describe the two mechanisms through which the ductus arteriosus closes after birth

Answer 9

Within hours:

1. The backflow of now oxygen-rich blood from the aorta into the ductus arteriosus causes a local increase in PaO2, causing vasoconstriction.
2. Reduction in placental production of prostaglandins also encourages closure of the ductus arteriosus

Question 10

By what week is the four-chambered heart developed?

Answer 10

Week 8

Question 11

What is the ductus venosus?

Answer 11

A venous shunt that allows half of the umbilical blood flow to bypass the liver and enter the IVC.

Question 12

Is the RV wall or LV wall thicker in the fetal circulation?

Answer 12

RV wall is thicker

RV output cannot be increased by Starling’s mechanism (increase in RA pressure increases RV output).

Question 13

How does the foramen ovale close?

Answer 13

Aeration of lungs increases oxygen tension and decreases pulmonary vascular resistance which leads to simultaneous:

• Decrease in RA afterload and RA pressure
• Increase in pulmonary flow and increased venous return to LA and increases LA pressure.

LA now > RA pressure leads to closure of the foramen ovale.

Question 14

Arginine vasopressin (AVP) also known as anti-diuretic hormone (ADH):

What is its physiological role?
In the fetus, what stimulus triggers increased secretion of AVP?

Answer 14

AVP/ADH role:

• Increase reabsorption of solute-free water from the renal tubules
• Constrict arterioles leading to increased peripheral vascular resistance and BP.

Secretion triggered in fetus by:

• Hypoxaemia (most potent stimulus)
• Decreased fetal intravascular volume or systemic BP.

Question 15

What % of fetal Hb is HbA type at term?

What % of fetal Hb if HbF type at term?

Answer 15

HbA 30%

HbF 70% (down from 100% earlier in pregnancy)