KEY POINTS

1. The fetus depends on the placenta for exchange of gas, nutrients, and metabolic byproducts.

2. The efficiency of gas exchange at the placenta depends on the species-variable arrangement of fetal and maternal blood vessels.

3. The fetal circulation mixes oxygenated and deoxygenated blood at several points, so the fetus exists in a state of hypoxemia.

4. Fetal oxygen transport is assisted by fetal hemoglobin, which has a high affinity for oxygen.

5. The lung develops in three stages, and pulmonary surfactant must be present at birth.

6. At or shortly after birth, umbilical vessels rupture, pulmonary vascular resistance decreases, and the foramen ovale and ductus arteriosus close.