Which of the following are functions of the placenta in fetal circulation? Select all that apply.

Oxygen affinity of fetal hemoglobin is regulated by molecular structure and interaction with intracellular modulators such as 2,3-bisphosphoglycerate (2,3-BPG). Differences in subunit composition, oxygen binding dynamics, hemoglobin dissociation curve, and blood saturation levels explain why fetal hemoglobin is better suited for placental gas exchange. HbF binds oxygen more tightly than HbA due to reduced sensitivity to 2,3-BPG; PaO₂ in umbilical venous blood is typically 30–35 mmHg with oxygen saturation around 70–80%.

Rationale for correct answers

B. Fetal hemoglobin (HbF) is composed of two alpha and two gamma chains (α₂γ₂), unlike adult hemoglobin (HbA) which has two alpha and two beta chains (α₂β₂). The γ chains have reduced affinity for 2,3-BPG, an allosteric modulator that normally decreases hemoglobin's oxygen affinity. Less binding of 2,3-BPG allows HbF to retain higher oxygen affinity, critical for extracting oxygen from maternal blood across the placenta.

Rationale for incorrect answers

A. Hemoglobin size does not influence oxygen affinity. Both HbF and HbA have tetrameric structures and comparable molecular weight (~64 kDa). Oxygen binding characteristics are not determined by molecular size but by subunit interactions and responsiveness to modulators like 2,3-BPG.

C. Carbon dioxide transport is mediated through different mechanisms including carbamino formation and bicarbonate buffering. HbF does not demonstrate superior carbon dioxide carriage over HbA. The increased oxygen affinity of HbF is unrelated to carbon dioxide handling.

D. While HbF concentration in fetal blood is higher than HbA concentration in adults (about 70–90% of total hemoglobin in the fetus), oxygen affinity is not determined by quantity. It is a function of molecular structure and reduced modulation by 2,3-BPG. Concentration influences oxygen-carrying capacity, not affinity.

Take home points

• HbF’s α₂γ₂ structure leads to lower 2,3-BPG binding and higher oxygen affinity.
• HbF is adapted for efficient oxygen uptake from low PaO₂ placental blood.
• Oxygen affinity is determined by molecular composition, not hemoglobin concentration.
• HbF's oxygen affinity facilitates fetal survival in relatively hypoxic intrauterine conditions.

Ductus venosus is the fetal vascular shunt that bypasses the liver, allowing oxygenated blood from the umbilical vein to flow directly into the inferior vena cava (IVC). This adaptation ensures that the most highly oxygenated blood (with an oxygen saturation around 80%) reaches the heart and brain quickly without undergoing hepatic filtration. In fetal circulation, the liver receives only a small portion of the oxygenated umbilical blood, as the ductus venosus diverts the majority. After birth, this shunt closes functionally within minutes to hours, with complete anatomical closure within 1–2 weeks, becoming the ligamentum venosum.

Rationale for correct answers

C. The ductus venosus is the fetal shunt that connects the umbilical vein to the IVC, bypassing the liver. It allows the majority of oxygen-rich placental blood to enter the systemic circulation directly, prioritizing perfusion to critical fetal organs.

Rationale for incorrect answers

A. The foramen ovale is an atrial-level shunt that bypasses the fetal lungs by directing blood from the right atrium to the left atrium. It plays no role in hepatic circulation.

B. The ductus arteriosus connects the pulmonary artery to the descending aorta, bypassing the lungs. It does not affect liver perfusion.

D. The ligamentum teres is the fibrous remnant of the umbilical vein after birth. It does not function as a shunt and is not present during fetal circulation as a bypass mechanism.

Take home points

• The ductus venosus bypasses the fetal liver by shunting blood to the IVC.
• It allows rapid delivery of oxygenated blood from the placenta to the heart.
• Postnatal closure forms the ligamentum venosum.
• Other fetal shunts include foramen ovale (atria) and ductus arteriosus (pulmonary artery to aorta).