Question

Which is the correct statement regarding fetal circulation?

• A. Umbilical venous blood completely bypasses the liver
• B. Blood from the IVC flows from the RA to the LA via the Foramen Ovale
• C. Umbilical vein saturation 30-35%
• D. SVC blood drains into the left atrium

Hint:

Fetal streaming: High-oxygen IVC stream ($\text{O}_2 \approx 70\%$) $\rightarrow$ Foramen Ovale $\rightarrow$ LA $\rightarrow$ Brain/Heart. Low-oxygen SVC stream ($\text{O}_2 \approx 40\%$) $\rightarrow$ RV $\rightarrow$ Pulmonary Trunk $\rightarrow$ Ductus Arteriosus $\rightarrow$ Lower Body.

Answer 1

The Correct Option is B

Concept:
The fetal circulation is uniquely adapted to receive oxygenated, nutrient-rich blood from the placenta and direct it preferentially toward developing vital organs (the brain and myocardium) through specialized fetal shunts.
These key anatomical shunts include the ductus venosus, foramen ovale, and ductus arteriosus.
Explanation:
• Well-oxygenated blood ($80$--$85\%$ oxygen saturation) enters the fetus from the placenta via the single umbilical vein.

• Approximately $50\%$ of umbilical venous blood enters the hepatic microcirculation to perfuse the liver, while the remaining $50\%$ bypasses the liver parenchyma via the ductus venosus directly into the inferior vena cava (IVC).

• Oxygenated IVC blood reaches the right atrium (RA) where the Eustachian valve and the crista dividens preferentially streamline and shunt the majority of this high-velocity flow across the foramen ovale into the left atrium (LA).

• From the LA, oxygen-rich blood passes to the left ventricle (LV) and is ejected into the ascending aorta to preferentially supply the cerebral and coronary circulations.

• Superior vena cava (SVC) blood is deoxygenated ($30$--$40\%$ saturation) and drains directly through the tricuspid valve into the right ventricle (RV), not the left atrium, and is subsequently pumped into the pulmonary artery and across the ductus arteriosus.
Final Answer:
The correct statement is that blood from the IVC is preferentially directed from the right atrium across the foramen ovale into the left atrium.