Unlike the adult whose lungs handle oxygen exchange, the foetus relies entirely on the placenta for survival. The foetal lungs are collapsed, filled with fluid, and have very high vascular resistance. Oxygen and nutrient exchange happen through maternal blood bathing the chorionic villi without direct mixing of maternal and foetal blood.
Blood returning from the placenta via the umbilical vein enters the foetal body, heading toward the liver. However, the liver does not require the entire volume during foetal life.
About half of this oxygenated blood bypasses the hepatic circulation through the ductus venosus, a specialized shunt vessel. The ductus venosus drains directly into the inferior vena cava, mixing with deoxygenated blood coming from the lower limbs and abdominal organs. This mixture then enters the right atrium of the heart, where preferential streaming directs it toward vital upper body structures.
The foetal heart features two critical shunts designed to divert blood away from the non-functioning pulmonary circuit.
Foetal circulation is characterized by parallel circuits rather than the series circuits seen in adults. The blood leaving the heart is mixed, but the streaming effect ensures that the brain and coronary circulation receive blood with the highest available oxygen saturation. Deoxygenated blood from the upper body returns via the superior vena cava into the right atrium, passes into the right ventricle, and exits through the pulmonary artery, where the ductus arteriosus guides it into the descending aorta toward the placenta for reoxygenation.
Foetal haemoglobin ($ ext{HbF}$) has a higher affinity for oxygen than adult haemoglobin ($ ext{HbA}$), shifting the oxygen dissociation curve to the left. This adaptation allows the foetus to effectively extract oxygen from maternal blood across the placental membrane. Additionally, pulmonary vascular resistance is extremely high in the foetus because the lungs are uninflated, whereas systemic vascular resistance is low due to the low-resistance placental bed.
When a newborn takes its first breath, dramatic circulatory adjustments occur. Lung expansion causes pulmonary vascular resistance to drop sharply, increasing blood flow through the lungs. Clamping the umbilical cord removes the low-resistance placental bed, causing systemic vascular resistance to rise.