Final answer:
Fetal circulation differs from postnatal circulation primarily because the fetal lungs are not used for gas exchange. Instead, oxygen is obtained from the mother's blood via the placenta, with fetal hemoglobin ensuring efficient oxygen transfer. After birth, shunts close, and the lungs take over the role of oxygenation.
Step-by-step explanation:
The major differences between fetal circulation and normal postnatal circulation stem from the unique requirements of the fetal environment. In fetal circulation, the lungs are not yet functional, so blood bypasses this system via two key shunts: the foramen ovale and the ductus arteriosus. These shunts redirect blood away from the lungs and towards the placenta, where gas exchange occurs. After birth, these shunts close, and the lungs become the primary site for oxygen and carbon dioxide exchange.
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, enabling efficient transfer of oxygen from the mother's blood to the fetus through the placenta. This is a critical adaptation because the fetus must obtain oxygen in an environment where it cannot breathe air.