Fetal Circulatory System

The fetal circulatory system is beautifully adapted to bypass the lungs and liver while maximizing placental exchange. Blood leaves the fetus through two umbilical arteries branching from the iliac arteries, carrying deoxygenated blood and waste to the placenta, where it picks up oxygen and nutrients in maternal sinusoids without direct vessel mixing. Highly oxygenated blood returns via the single umbilical vein to the liver, where about half perfuses hepatic tissue and half streams through the ductus venosus into the inferior vena cava. Preferential flow patterns in the heart ensure the best-oxygenated blood reaches the brain and heart first, while lower-body tissues receive more mixed blood. These shunts—foramen ovale, ductus arteriosus, and ductus venosus—close after birth in response to pressure changes and oxygen levels, completing the switch to separate pulmonary and systemic circulations.