Fetal Circulation

Fetal Hemoglobin (HbF)

A special form of hemoglobin (2 alpha, 2 gamma chains) with a higher oxygen affinity than adult hemoglobin (HbA).

Oxygen affinity of HbF

Higher than that of adult hemoglobin (HbA) to 'steal' oxygen from maternal blood at the placenta.

P50 value of HbF vs HbA

HbF has a P50 of 20 mmHg, HbA has a P50 of 27 mmHg.

Hematopoiesis in a fetus

Begins in the liver and spleen (early); bone marrow and lymph nodes take over later.

Cause of infant jaundice

Rapid breakdown of HbF after birth leads to excess bilirubin buildup.

Percentage of HbF at birth

About 80%.

Percentage of HbA by 6–12 months

Around 90%.

Oxygen dissociation curve shift after birth

Shifts right, indicating HbA releases oxygen more easily to tissues.

Fetal heart pumping start

Around 3.5 weeks gestation.

Fetal heart with 4 chambers development

By 7 weeks gestation.

Purpose of fetal circulation shunts

To bypass the lungs, which are not yet used for oxygen exchange.

Foramen ovale

A hole between the right and left atria that allows blood to bypass the lungs.

Ductus arteriosus

A vessel that connects the pulmonary artery to the aorta.

Ductus venosus

A vessel that bypasses the liver, sending blood directly to the heart.

Function of the placenta

Acts as lungs, kidneys, and digestive system for the fetus—gas exchange, waste removal, and nutrient delivery.

Transplacental barrier

A semi-permeable membrane that separates fetal and maternal blood.

Substances that can cross the placental barrier

Gases (O₂, CO₂), small nutrients, antibodies, some medications/drugs.

Placenta efficiency despite smaller surface area

HbF holds oxygen tightly, fetal blood has more hemoglobin, and a strong oxygen gradient from mom to baby.

Trigger for circulatory change after birth

Baby's first breath inflates lungs and drops pulmonary resistance.

Fetal shunt changes after birth

Foramen ovale: closes functionally at birth, anatomically within months; Ductus arteriosus: functionally closes in hours, anatomically in days.

Cause of foramen ovale closure

Increased pressure in the left atrium due to lung use and loss of placenta.