Safety margin of fetal oxygenation

The supply of O2 to fetal tissues depends upon a balance between factors that favor and hinder fetal oxygenation. Experiments in sheep have demonstrated that high rates of placental perfusion, high O2 affinity of fetal red cells and high rates of perfusion in fetal tissues compensate for an ineffective placental exchange system and for the circulatory arrangement by which fetal arterial blood is formed by mixing oxygenated umbilical venous blood with deoxygenated systemic venous blood. Under normal physiologic conditions the balance of the factors that influence fetal oxygenation provides the fetal lamb with an oxygen supply approximately twice the minimum level necessary to sustain fetal life.