Question

Which of the following statements regarding fetal circulation is correct?

• A. The liver and heart of the fetus receive blood with very high oxygen saturation
• B. PO2 of fetal blood leaving the placenta is slightly greater than maternal mixed venous PO2
• C. The presence of fetal hemoglobin shifts the oxyhemoglobin dissociation to the right
• D. The foramen ovale closes during the third trimester unless the fetus has an atrial septal defect

Hint:

HbF shifts the curve left and the foramen ovale stays open until birth.

Answer 1

The Correct Option is A

Step 1: Oxygenated blood returns from the placenta in the umbilical vein. Part of it perfuses the liver and the rest joins the inferior vena cava via the ductus venosus. This stream then reaches the heart and, through preferential streaming across the foramen ovale, supplies the head and coronary circulation before mixing with desaturated blood. So the liver, heart and brain receive the best oxygenated blood. Option (a) is correct.

Step 2: Option (b) is wrong. Because of high placental blood flow and the resistance of the placenta to oxygen diffusion, umbilical vein PO2 is only about 30 mm Hg, which is lower than maternal mixed venous PO2, not higher.

Step 3: Option (c) is wrong. Fetal hemoglobin (HbF) has a higher oxygen affinity and shifts the oxyhemoglobin dissociation curve to the left, not the right. This left shift, together with the Bohr effect, helps load oxygen onto fetal blood at the placenta.

Step 4: Option (d) is wrong. The foramen ovale stays open throughout fetal life and normally closes functionally only after birth, when left atrial pressure rises above right atrial pressure. Hence the only true statement is (a).